code: public synchronized @Nullable CloseableReference<Bitmap> getDecodedFrame(int index) {
      if (mDecodedFrames != null) {
        return CloseableReference.cloneOrNull(mDecodedFrames.get(index));
      }
      return null;
    }
docs: Gets a decoded frame. This will only return non-null if the {@code ImageDecodeOptions}
  were configured to decode all frames at decode time.
  
  @param index the index of the frame to get
  @return a reference to the preview bitmap which must be released by the caller when done or
  null if there is no preview bitmap set