Loads a bitmap from an async task.

LoadBitmapAsyncTask.java

@Override
public void getThumbnail(int index, ResultCallback callback) {
   LoadBitmapAsyncTask task = new LoadBitmapAsyncTask(index, callback);
   task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
}

class LoadBitmapAsyncTask extends AsyncTask<Void, Void, Bitmap> {

       final int mIndex;
       final ResultCallback mResultCallback;

       public LoadBitmapAsyncTask(int index, ResultCallback resultCallback) {
           mIndex = index;
           mResultCallback = resultCallback;
       }

       @Override
       protected Bitmap doInBackground(Void... voids) {
		   MediaMetadataRetriever retriever = new MediaMetadataRetriever();
		   retriever.setDataSource(mVideoUrl, new HashMap<>());
		   long position = getSeekPositions()[index];
		   Log.d("SeekProvider", "position: " + position);

		   Bitmap thumbnail = retriever.getFrameAtTime(position, 
						MediaMetadataRetriever.OPTION_CLOSEST);
		   retriever.release();
		   return thumbnail;
       }

       @Override
       protected void onPostExecute(Bitmap bitmap) {
           super.onPostExecute(bitmap);
           mResultCallback.onThumbnailLoaded(bitmap, mIndex);
       }
   }