Cache background tasks per index.

GetThumbnailAsync.java

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