[Mono.Android] Do not throw in JniValueManager

The new marshaling code uses `JniValueManager`, so we don't want to
trow here. Just keep it empty to make things work, until we have
proper implementation of it.

Author: radekdoulik

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

@@ -363,12 +363,10 @@ public override void WaitForGCBridgeProcessing ()
 
 		public override void AddPeer (IJavaPeerable value)
 		{
-			throw new NotImplementedException ();
 		}
 
 		public override void RemovePeer (IJavaPeerable value)
 		{
-			throw new NotImplementedException ();
 		}
 
 		public override IJavaPeerable PeekPeer (JniObjectReference reference)
@@ -378,17 +376,15 @@ public override IJavaPeerable PeekPeer (JniObjectReference reference)
 
 		public override void CollectPeers ()
 		{
-			throw new NotImplementedException ();
 		}
 
 		public override void FinalizePeer (IJavaPeerable value)
 		{
-			throw new NotImplementedException ();
 		}
 
 		public override List<JniSurfacedPeerInfo> GetSurfacedPeers ()
 		{
-			throw new NotImplementedException ();
+			return null;
 		}
 	}
 }