Fix AudioTrack::flush()

It was only flushing at a surface level, and even then only the first time
the server observed the client's flush request.  Now it flushes at a
deeper level, but there may be even deeper device-specific flushing.

Bug: 9770947
Change-Id: I687cc3410ff9e5e5d4a5dcb9e3b129501e53d247