fixes for Android transparency and add removeAsset method

android/src/main/cpp/filament_android.cpp

@@ -169,5 +169,9 @@ extern "C" {
   void release_source_assets(void* viewer) {
     ((FilamentViewer*)viewer)->releaseSourceAssets();
   }
+
+  void remove_asset(void* viewer) {
+    ((FilamentViewer*)viewer)->removeAsset();
+  }
   
 }