[Mono.Android] fix a set of the "easiest" trimmer warnings (dotnet#8731)

Context: dotnet#5652

Fixing the following trimmer warnings:

~~ TypeConverter ~~

Context: https://source.dot.net/#System.ComponentModel.TypeConverter/System/ComponentModel/TypeConverter.cs,226

	src\Mono.Android\System.Drawing\SizeFConverter.cs(121,49): error IL2046:
	  Base member 'System.ComponentModel.TypeConverter.GetProperties(ITypeDescriptorContext, Object, Attribute[])' with 'RequiresUnreferencedCodeAttribute' has a derived member
	  'System.Drawing.SizeFConverter.GetProperties(ITypeDescriptorContext, Object, Attribute[])' without 'RequiresUnreferencedCodeAttribute'.
	  'RequiresUnreferencedCodeAttribute' annotations must match across all interface implementations or overrides.

Various `TypeConverter` implementations need to specify
`[RequiresUnreferencedCode]` to match the base type.


~~ ColorValueMarshaler & IJavaObjectValueMarshaler ~~

Context: dotnet/java-interop@7d1e705

From the trimmer warnings solved in `Java.Interop.dll`, we need to
bring these changes forward to any `*Marshaler` types.


~~ AndroidClientHandler ~~

	'this' argument does not satisfy 'DynamicallyAccessedMemberTypes.NonPublicFields' in call to 'System.Type.GetField(String, BindingFlags)'.
	The return value of method 'System.Collections.IEnumerator.Current.get' does not have matching annotations.
	The source value must declare at least the same requirements as those declared on the target location it is assigned to.

This class had a loop that was not trimming safe:

	for (var type = GetType (); type != null; type = type.BaseType) {
	    field = type.GetField (fieldName, BindingFlags.Instance | BindingFlags.NonPublic);
	    if (field != null)
	        break;
	}

If we *look* at the actual base types of `AndroidClientHandler`, we
can simplify this loop to something that *is* trimming safe:

	const BindingFlags flags = BindingFlags.Instance | BindingFlags.NonPublic;
	FieldInfo? field = typeof (HttpClientHandler).GetField (fieldName, flags) ??
	    typeof (HttpMessageHandler).GetField (fieldName, flags);

There should be no need to iterate *beyond* `HttpMessageHandler`, the
code is looking for this field:

  * https://github.com/dotnet/runtime/blob/135fec006e727a31763271984cd712f1659ccbd3/src/libraries/System.Net.Http/src/System/Net/Http/HttpClientHandler.AnyMobile.cs#L25

As `AndroidClientHandler` is `[Obsolete]` and unlikely to have its
inheritance hierarchy changed, removing the loop is deemed safe.

src/Mono.Android/Android.Graphics/Color.cs

@@ -395,11 +395,18 @@ public static void RGBToHSV (int red, int green, int blue, float[] hsv)
 
 	public class ColorValueMarshaler : JniValueMarshaler<Color>
 	{
+		const DynamicallyAccessedMemberTypes ConstructorsAndInterfaces = DynamicallyAccessedMemberTypes.PublicConstructors | DynamicallyAccessedMemberTypes.NonPublicConstructors | DynamicallyAccessedMemberTypes.Interfaces;
+		const string ExpressionRequiresUnreferencedCode = "System.Linq.Expression usage may trim away required code.";
+
 		public override Type MarshalType {
 			get { return typeof (int); }
 		}
 
-		public override Color CreateGenericValue (ref JniObjectReference reference, JniObjectReferenceOptions options, Type targetType)
+		public override Color CreateGenericValue (
+				ref JniObjectReference reference,
+				JniObjectReferenceOptions options,
+				[DynamicallyAccessedMembers (ConstructorsAndInterfaces)]
+				Type targetType)
 		{
 			throw new NotImplementedException ();
 		}
@@ -414,6 +421,7 @@ public override void DestroyGenericArgumentState (Color value, ref JniValueMarsh
 			throw new NotImplementedException ();
 		}
 
+		[RequiresUnreferencedCode (ExpressionRequiresUnreferencedCode)]
 		public override Expression CreateParameterToManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue, ParameterAttributes synchronize, Type targetType)
 		{
 			var c = typeof (Color).GetConstructor (new[]{typeof (int)})!;
@@ -424,6 +432,7 @@ public override Expression CreateParameterToManagedExpression (JniValueMarshaler
 			return v;
 		}
 
+		[RequiresUnreferencedCode (ExpressionRequiresUnreferencedCode)]
 		public override Expression CreateParameterFromManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue, ParameterAttributes synchronize)
 		{
 			var r = Expression.Variable (MarshalType, sourceValue.Name + "_p");
@@ -433,6 +442,7 @@ public override Expression CreateParameterFromManagedExpression (JniValueMarshal
 			return r;
 		}
 
+		[RequiresUnreferencedCode (ExpressionRequiresUnreferencedCode)]
 		public override Expression CreateReturnValueFromManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue)
 		{
 			return CreateParameterFromManagedExpression (context, sourceValue, 0);

src/Mono.Android/Android.Runtime/IJavaObjectValueMarshaler.cs

@@ -10,9 +10,16 @@ namespace Android.Runtime
 {
 	sealed class IJavaObjectValueMarshaler : JniValueMarshaler<IJavaObject> {
 
+		const DynamicallyAccessedMemberTypes ConstructorsAndInterfaces = DynamicallyAccessedMemberTypes.PublicConstructors | DynamicallyAccessedMemberTypes.NonPublicConstructors | DynamicallyAccessedMemberTypes.Interfaces;
+		const string ExpressionRequiresUnreferencedCode = "System.Linq.Expression usage may trim away required code.";
+
 		internal    static  IJavaObjectValueMarshaler              Instance    = new IJavaObjectValueMarshaler ();
 
-		public override IJavaObject CreateGenericValue (ref JniObjectReference reference, JniObjectReferenceOptions options, Type? targetType)
+		public override IJavaObject CreateGenericValue (
+				ref JniObjectReference reference,
+				JniObjectReferenceOptions options,
+				[DynamicallyAccessedMembers (ConstructorsAndInterfaces)]
+				Type? targetType)
 		{
 			throw new NotImplementedException ();
 		}
@@ -27,6 +34,7 @@ public override void DestroyGenericArgumentState ([AllowNull]IJavaObject value,
 			throw new NotImplementedException ();
 		}
 
+		[RequiresUnreferencedCode (ExpressionRequiresUnreferencedCode)]
 		public override Expression CreateReturnValueFromManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue)
 		{
 			return Expression.Call (
@@ -36,6 +44,7 @@ public override Expression CreateReturnValueFromManagedExpression (JniValueMarsh
 				sourceValue);
 		}
 
+		[RequiresUnreferencedCode (ExpressionRequiresUnreferencedCode)]
 		public override Expression CreateParameterToManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue, ParameterAttributes synchronize, Type? targetType)
 		{
 			var r   = Expression.Variable (targetType, sourceValue.Name + "_val");

src/Mono.Android/System.Drawing/PointConverter.cs

@@ -30,6 +30,7 @@
 
 using System.Collections;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.ComponentModel.Design.Serialization;
 using System.Runtime.CompilerServices;
@@ -133,6 +134,7 @@ public override bool GetCreateInstanceSupported (ITypeDescriptorContext context)
 			return true;
 		}
 
+		[RequiresUnreferencedCode ("The Type of value cannot be statically discovered.")]
 		public override PropertyDescriptorCollection? GetProperties (
 							ITypeDescriptorContext context,
 							object value, Attribute[] attributes)

src/Mono.Android/System.Drawing/RectangleConverter.cs

@@ -31,6 +31,7 @@
 
 using System.ComponentModel;
 using System.Collections;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Text;
 using System.ComponentModel.Design.Serialization;
@@ -147,6 +148,7 @@ public override bool GetCreateInstanceSupported (ITypeDescriptorContext context)
 			return true;
 		}
 
+		[RequiresUnreferencedCode ("The Type of value cannot be statically discovered.")]
 		public override PropertyDescriptorCollection? GetProperties (
 							ITypeDescriptorContext context,
 							object value, Attribute[] attributes)

src/Mono.Android/System.Drawing/SizeConverter.cs

@@ -31,6 +31,7 @@
 
 using System.Collections;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.ComponentModel.Design.Serialization;
 using System.Reflection;
@@ -135,6 +136,7 @@ public override bool GetCreateInstanceSupported (ITypeDescriptorContext context)
 			return true;
 		}
 
+		[RequiresUnreferencedCode ("The Type of value cannot be statically discovered.")]
 		public override PropertyDescriptorCollection? GetProperties (
 							ITypeDescriptorContext context,
 							object value, Attribute[] attributes)

src/Mono.Android/System.Drawing/SizeFConverter.cs

@@ -32,6 +32,7 @@
 using System;
 using System.Collections;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.ComponentModel.Design.Serialization;
 using System.Reflection;
@@ -118,6 +119,7 @@ public override bool GetCreateInstanceSupported (ITypeDescriptorContext context)
 			return true;
 		}
 
+		[RequiresUnreferencedCode ("The Type of value cannot be statically discovered.")]
 		public override PropertyDescriptorCollection? GetProperties (ITypeDescriptorContext context, object value, Attribute[] attributes)
 		{
 			if (value is SizeF)

src/Mono.Android/Xamarin.Android.Net/AndroidClientHandler.cs

@@ -313,14 +313,9 @@ protected virtual Task SetupRequest (HttpRequestMessage request, HttpURLConnecti
 		object? GetUnderlyingHandler ()
 		{
 			var fieldName = "_nativeHandler";
-			FieldInfo? field = null;
-
-			for (var type = GetType (); type != null; type = type.BaseType) {
-				field = type.GetField (fieldName, BindingFlags.Instance | BindingFlags.NonPublic);
-				if (field != null)
-					break;
-			}
-
+			const BindingFlags flags = BindingFlags.Instance | BindingFlags.NonPublic;
+			FieldInfo? field = typeof (HttpClientHandler).GetField (fieldName, flags) ??
+				typeof (HttpMessageHandler).GetField (fieldName, flags);
 			if (field == null) {
 				throw new InvalidOperationException ($"Field '{fieldName}' is missing from type '{GetType ()}'.");
 			}