[Java.Interop] Add JniRuntime.JniValueManager.GetPeer()

[jonpryor]

Context: https://github.com/dotnet/android/pull/9630/files#r1891090085
Context: dotnet/android#9716

As part of NativeAOT prototyping within dotnet/android, we need to update Java.Lang.Object.GetObject<T>() so that it uses Java.Interop.JniRuntime.JniValueManager APIs instead of its own TypeManager.CreateInstance() invocation, as
TypeManager.CreateInstance() hits P/Invokes which don't currently work in the NativeAOT sample environment.

However, updated it to what? The obvious thing to do would be to use JniRuntime.JniValueManager.GetValue():

partial class Object {
    internal static IJavaPeerable? GetObject (IntPtr handle, JniHandleOwnershipt ransfer, Type? type = null)
    {
        var r = PeekObject (handle, type);
        if (r != null) {
            JNIEnv.DeleteRef (handle, transfer);
            return r;
        }
        var reference = new JniObjectReference (handle);
        r = (IJavaPeerable) JNIEnvInit.ValueManager.GetValue (
                ref reference,
                JniObjectReferenceOptions.Copy,
                type);
        JNIEnv.DeleteRef (handle, transfer);
        return r;
    }
}

The problem is that this blows up good:

<System.InvalidCastException: Arg_InvalidCastException
   at Java.Lang.Object.GetObject(IntPtr , JniHandleOwnership , Type )
   at Android.Runtime.JNIEnv.<CreateNativeArrayElementToManaged>g__GetObject|74_11(IntPtr , Type )
   at Android.Runtime.JNIEnv.<>c.<CreateNativeArrayElementToManaged>b__74_9(Type type, IntPtr source, Int32 index)
   at Android.Runtime.JNIEnv.GetObjectArray(IntPtr , Type[] )
   at Java.InteropTests.JnienvTest.<>c__DisplayClass26_0.<MoarThreadingTests>b__1()>

because somewhere in that stack trace we have a java.lang.Integer instance, and .GetValue(Integer_ref, …) returns a System.Int32 containing the underling value, not an IJavaPeerable value for the java.lang.Integer instance. Consider:

var i_class = new JniType ("java/lang/Integer");
var i_ctor  = i_class.GetConstructor ("(I)V");
JniArgumentValue* i_args = stackalloc JniArgumentValue [1];
i_args [0]  = new JniArgumentValue (42);
var i_value = i_class.NewObject (i_ctor, i_args);
var v       = JniEnvironment.Runtime.ValueManager.GetValue (ref i_value, JniObjectReferenceOptions.CopyAndDispose, null);
Console.WriteLine ($"v? {v} {v?.GetType ()}");

which prints v? 42 System.Int32.

This was expected and desirable, until we try to use GetValue() for Object.GetObject<T>(); the semantics don't match.

Add a new JniRuntime.JniValueManager.GetPeer() method, which better matches the semantics that Object.GetObject<T>() requires, allowing:

partial class Object {
    internal static IJavaPeerable? GetObject (IntPtr handle, JniHandleOwnershipt ransfer, Type? type = null)
    {
        var r = JNIEnvInit.ValueManager.GetPeer (new JniObjectReference (handle));
        JNIEnv.DeleteRef (handle, transfer);
        return r;
    }
}

Finally, add a new JniRuntimeJniValueManagerContract unit test, so that we have "more formalized" semantic requirements on JniRuntime.JniValueManager implementations.

Copilot reviewed 2 out of 5 changed files in this pull request and generated no comments.

Files not reviewed (3)

• src/Java.Interop/PublicAPI.Unshipped.txt: Language not supported
• tests/Java.Interop-Tests/Java.Interop-Tests.csproj: Language not supported
• tests/TestJVM/TestJVM.csproj: Language not supported

[jonpryor]

/azp run

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[jonathanpeppers]

I was able to update Java.Lang.Object.GetObject() to use this method:

var p = JNIEnvInit.ValueManager!.GetPeer (new JniObjectReference (handle), type);
JNIEnv.DeleteRef (handle, transfer);
return p;

After merging this, we can try it in a new version of:

• [Mono.Android] Java.Lang.Object.GetObject<T>() implementation android#9630

[jonpryor]

/azp run

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