[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!

This fails to build with:

    src\Mono.Android\Android.Runtime\AndroidRuntime.cs(78,60): error CS0122: 'JniRuntime.JniValueManager.TryUnboxPeerObject(IJavaPeerable, out object?)' is inaccessible due to its protection level

Author: jonathanpeppers

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

@@ -75,8 +75,8 @@ public override string GetCurrentManagedThreadStackTrace (int skipFrames, bool f
 
 		Exception? UnboxException (IJavaPeerable value)
 		{
-			if (JNIEnvInit.ValueManager is AndroidValueManager vm) {
-				return vm.UnboxException (value);
+			if (JNIEnvInit.ValueManager is JniValueManager vm && vm.TryUnboxPeerObject (value, out var r) && r is Exception e) {
+				return e;
 			}
 			return null;
 		}
@@ -865,15 +865,6 @@ 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);
+	}
 }