@Nullable
  public synchronized CloseableReference<Bitmap> getPreviewBitmap() {
    return CloseableReference.cloneOrNull(mPreviewBitmap);
  }
Gets the bitmap for the preview frame. This will only return non-null if the {@code
ImageDecodeOptions} were configured to decode the preview frame.

@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