@Nullable
  public V pop() {
    return (V) mFreeList.poll();
  }
Remove the first item (if any) from the freelist. Returns null if the free list is empty Does
not update the bucket inUse count

@return the first value (if any) from the free list
imagepipeline/src/main/java/com/facebook/imagepipeline/memory/Bucket.java