[Java.Interop-Tests] Initial dotnet test support

[jonpryor]

Context: dotnet/android#5592

With introductory support for Java.Interop.dll on .NET Core via
commit 2a299eb, what about running the unit tests within
Java.Interop-Tests.dll on .NET Core?

Update src/java-interop so that the java-interop native library
is copied to $(OutputPath) of referencing projects. This allows
e.g. libjava-interop.dylib to be implicitly copied into the
appropriate output dir, e.g. bin/TestDebug-netcoreapp3.1.
Further update src/java-interop so that $(IntermediateOutputPath)
correctly differs between net472 and netcoreapp3.1 builds, and
generate jni.c into $(IntermediateOutputPath). This ensures that
e.g. bin/Debug/libjava-interop.dylib reliably uses Mono on net472,
instead of occasionally/accidentally being the netstandard version.

Update tests/Java.Interop-Tests and tests/TestJVM/TestJVM.csproj
to multitarget between net472 and netcoreapp3.1.

Update Java.Runtime.Environment.csproj to reference
java-interop.csproj, which causes all projects which reference
Java.Runtime.Environment.csproj to get the java-interop native
library copied to the appropriate $(OutputPath).

Update JreRuntime to use "dummy" JniRuntime.JniValueManager and
JniRuntime.JniObjectReferenceManager subclasses when not running
under Mono. This avoids an ArgumentException when creating the
Java.InteropTests.JavaVMFixture instance used by the unit tests.

Update tests/TestJVM to reference
Xamarin.Android.Tools.AndroidSdk.csproj, allowing the TestJVM type
to use JdkInfo to find a JVM to use. This allows us to avoid a
requirement to set the JI_JVM_PATH environment variable in order to
create TestJVM instances.

Update src/Java.Interop so that generic delegate types are not
marshaled via P/Invoke. .NET Core doesn't support this.

Update JniEnvironment.Types.FindClass() to call
info.Runtime.GetExceptionForThrowable() as soon as possible after
calling JNIEnv::ExceptionClear(), instead of potentially after
calling ClassLoader.loadClass(). The problem is Java-side stack-
frames: if info.Runtime.GetExceptionForThrowable() is delayed, then
the exception created by info.Runtime.GetExceptionForThrowable()
only contains the Java message text, but no stack trace, and no
"cause" information. This meant that when generic delegate types
were still being used, dotnet test failures would show:

    java.lang.NoClassDefFoundError: Could not initialize class com.xamarin.interop.CallVirtualFromConstructorDerive

with no additional information about why it failed.
(It failed because P/Invoke was trying to marshal a generic delegate,
so the ManagedPeer static constructor threw an exception, which
was "within" a static{ManagedPeer.registerNativeMembers(…)} Java
block, and thus the Java type initializer failed, so nothing worked.)

Update **/*.java to remove Version, Culture, and PublicKeyToken
information from the assembly qualified name, for two reasons:

1. The default Version value appears to differ between
   net472 (0.0.0.0) and netcoreapp3.1 (1.0.0.0).

2. Wrt MobSf Scan Security issue android#5592, certain "security products"
   believe that the Version number is an IP address, and emit a
   warning about "IP Address disclosure".

Since we don't strong-name any of these assemblies (unit tests!)
or the expected execution environment is a "self-contained app"
in which you can't have multiple versions of the same assembly
(Xamarin.Android), remove the Version, Culture, and PublicKeyToken
information from these Java Callable Wrappers.

[jonpryor]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jonpryor]

Note that this is initial; it is not "complete":

% dotnet test bin/TestDebug-netcoreapp3.1/Java.Interop-Tests.dll
Failed!  - Failed:    16, Passed:   325, Skipped:     1, Total:   342, Duration: 310 ms - /Users/jon/Documents/Developer/src/xamarin/Java.Interop/bin/TestDebug-netcoreapp3.1/Java.Interop-Tests.dll (netcoreapp3.1)

16 tests pass, mostly because the DummyValueManager doesn't do anything, so all tests which requires it fail.