[Mono.Android] fix AndroidRuntime.UnboxException()

`Java.InteropTests.JavaExceptionTests.InnerExceptionIsNotAProxy()` fails with:

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

Can be fixed by using `JNIEnvInit.ValueManager?.PeekValue()` instead.

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 System.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);
+	}
 }