@Nullable
  public CloseableReference<CloseableImage> cache(
      int frameIndex, CloseableReference<CloseableImage> imageRef) {
    return mBackingCache.cache(keyFor(frameIndex), imageRef, mEntryStateObserver);
  }
Caches the image for the given frame index.

<p>Important: the client should use the returned reference instead of the original one. It is
the caller's responsibility to close the returned reference once not needed anymore.

@return the new reference to be used, null if the value cannot be cached
animated-base/src/main/java/com/facebook/imagepipeline/animated/impl/AnimatedFrameCache.java