private FrameKey keyFor(int frameIndex) {
    return new FrameKey(mImageCacheKey, frameIndex);
  }
Gets the image to be reused, or null if there is no such image.

<p>The returned image is the least recently used image that has no more clients referencing it,
and it has not yet been evicted from the cache.

<p>The client can freely modify the bitmap of the returned image and can cache it again without
any restrictions.
animated-base/src/main/java/com/facebook/imagepipeline/animated/impl/AnimatedFrameCache.java