Explicitly flush compress/encrypt stream before close

We're losing the stream footer for mysterious reasons; maybe this
will help even though supposedly close() is expected to flush pending
buffered writes anyway.  In particular, an explicit flush of the
compression stage will generate a SYNC_FLUSH point, which ordinary
deflate-at-close operation does not.

Bug 28056941

Change-Id: I96580411257932d7addb176c6672eae72a5ac6f7