A flush of a video decoder connected to a native window must reclaim

output buffers already queued for rendering before considering a flush
completed. Otherwise the decoder may not have enough output buffers to
continue decoding after the discontinuity and we'll never dequeue more
from the native window.

Change-Id: I42e275dc336568e180081c6d7c0dc05fc9637c79
related-to-bug: 8578467