public synchronized @Nullable CloseableReference<Bitmap> getDecodedFrame(int index) {
    if (mDecodedFrames != null) {
      return CloseableReference.cloneOrNull(mDecodedFrames.get(index));
    }
    return null;
  }
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
animated-base/src/main/java/com/facebook/imagepipeline/animated/base/AnimatedImageResult.java