[Mono.Android] fix "exception unboxing" for ManagedValueManager (#10100)

Context: #10095

After enabling `Java.Interop-Tests.dll` for CoreCLR/NativeAOT,
`Java.InteropTests.JavaExceptionTests.InnerExceptionIsNotAProxy()`
was failing with:

	Expected: same as <System.InvalidOperationException: Managed Exception!>
	But was:  <Java.Interop.JavaProxyThrowable: System.InvalidOperationException: Managed Exception!

This is because `AndroidRuntime.cs` was doing:

	if (JNIEnvInit.ValueManager is AndroidValueManager vm) {
	    return vm.UnboxException (value);
	}

And in this case `ManagedValueManager` was being used, and so the
exception would not be "unboxed".

We can fix this by using `JNIEnvInit.ValueManager?.PeekValue()`
instead and the test now passes.

I also noticed that `AndroidValueManager` had code in
`TryUnboxPeerObject()` for:

	var proxy = value as Android.Runtime.JavaProxyThrowable;
	if (proxy != null) {
	    result  = proxy.InnerException;
	    return true;
	}
	return base.TryUnboxPeerObject (value, out result);

While the base class handled `Java.Interop.JavaProxyThrowable`.
This was missing for `ManagedValueManager`, so I added it.

Author: jonathanpeppers

src/Mono.Android/Android.Runtime/AndroidRuntime.cs

@@ -66,21 +66,13 @@ public override string GetCurrentManagedThreadStackTrace (int skipFrames, bool f
 				return throwable;
 			}
 			JniObjectReference.Dispose (ref reference, options);
-			var unwrapped = UnboxException (peeked!);
+			var unwrapped = JNIEnvInit.ValueManager?.PeekValue (peeked!.PeerReference) as Exception;
 			if (unwrapped != null) {
 				return unwrapped;
 			}
 			return peekedExc;
 		}
 
-		Exception? UnboxException (IJavaPeerable value)
-		{
-			if (JNIEnvInit.ValueManager is AndroidValueManager vm) {
-				return vm.UnboxException (value);
-			}
-			return null;
-		}
-
 		public override void RaisePendingException (Exception pendingException)
 		{
 			var je  = pendingException as JavaException;
@@ -865,15 +857,6 @@ protected override bool TryUnboxPeerObject (IJavaPeerable value, [NotNullWhen (t
 			return base.TryUnboxPeerObject (value, out result);
 		}
 
-		internal Exception? UnboxException (IJavaPeerable value)
-		{
-			object? r;
-			if (TryUnboxPeerObject (value, out r) && r is Exception e) {
-				return e;
-			}
-			return null;
-		}
-
 		public override void CollectPeers ()
 		{
 			GC.Collect ();

src/Mono.Android/Microsoft.Android.Runtime/ManagedValueManager.cs

@@ -284,4 +284,14 @@ protected override bool TryConstructPeer (
 		}
 		return base.TryConstructPeer (self, ref reference, options, type);
 	}
+
+	protected override bool TryUnboxPeerObject (IJavaPeerable value, [NotNullWhen (true)]out object? result)
+	{
+		var proxy = value as JavaProxyThrowable;
+		if (proxy != null) {
+			result  = proxy.InnerException;
+			return true;
+		}
+		return base.TryUnboxPeerObject (value, out result);
+	}
 }