Properly parse acw-map.txt

`acw-map.txt` contains mappings from .NET types to Java types,
and implicitly vice-versa; see (TODO commit).

*Normally* it contains three entries:

 1. The fully-qualified .NET type name
 2. The .NET type name, no assembly
 3. (2) with a lowercased namespace name.

For example:

	Mono.Android_Test.Library.CustomTextView, Mono.Android-Test.Library.NET;crc6456ab8145c81c4100.CustomTextView
	Mono.Android_Test.Library.CustomTextView;crc6456ab8145c81c4100.CustomTextView
	mono.android_test.library.CustomTextView;crc6456ab8145c81c4100.CustomTextView

However, when XA4214 is emitted, there is a "collision" on the
.NET side (but *not* the Java side); (2) and (3) are *ambiguous*,
so one .NET type is arbitrarily chosen.

The first line is still possible, because of assembly qualification.

Enter ``Java.InteropTests.GenericHolder`1``: this type is present in
*both* `Java.Interop-Tests.dll` *and* `Mono.Android-Tests.dll`.
Before dotnet/java-interop#1181, this was "fine" because the
`GenericHolder<T>` within `Java.Interop-Tests.dll` did not participate
in typemap generation.  Now it does, resulting in the XA4214.
XA4214 *also* means that instead of three lines, it's *one* line:

	Java.InteropTests.GenericHolder`1, Mono.Android.NET-Tests;crc641855b07eca6dcc03.GenericHolder_1

Enter `<R8/>`, which parses `acw-map.txt` to create a
`proguard_project_primary.cfg` file.  `<R8/>` did it's *own* parsing
of `acw-map.txt`, parsing only *one of every three lines*, on the
assumption that *all* entries took three lines.

This breaks in the presence of XA4214, because some entries only take
one line, not three lines.

Update `<R8/>` to instead use `MonoAndroidHelper.LoadMapFile()`,
which reads all lines within `acw-map.txt`.  This should result in
a `proguard_project_primary.cfg` file which properly contains a
`-keep` entry for `crc641855b07eca6dcc03.GenericHolder_1`.

src/Xamarin.Android.Build.Tasks/Tasks/R8.cs

@@ -1,3 +1,4 @@
+using System;
 using Microsoft.Build.Framework;
 using Microsoft.Build.Utilities;
 using System.Collections.Generic;
@@ -82,16 +83,15 @@ protected override CommandLineBuilder GetCommandLineBuilder ()
 
 			if (EnableShrinking) {
 				if (!string.IsNullOrEmpty (AcwMapFile)) {
-					var acwLines = File.ReadAllLines (AcwMapFile);
+					var acwMap      = MonoAndroidHelper.LoadMapFile (BuildEngine4, Path.GetFullPath (AcwMapFile), StringComparer.OrdinalIgnoreCase);
+					var javaTypes   = new List<string> (acwMap.Values.Count);
+					foreach (var v in acwMap.Values) {
+						javaTypes.Add (v);
+					}
+					javaTypes.Sort (StringComparer.Ordinal);
 					using (var appcfg = File.CreateText (ProguardGeneratedApplicationConfiguration)) {
-						for (int i = 0; i + 2 < acwLines.Length; i += 3) {
-							try {
-								var line = acwLines [i + 2];
-								var java = line.Substring (line.IndexOf (';') + 1);
-								appcfg.WriteLine ("-keep class " + java + " { *; }");
-							} catch {
-								// skip invalid lines
-							}
+						foreach (var java in javaTypes) {
+							appcfg.WriteLine ($"-keep class {java} {{ *; }}");
 						}
 					}
 				}