AudioTrack: fix stall if setBufferSizeInFrames() called before play()

The server was waiting for a full buffer.
But the buffer was only getting partly filled.
So the stream was not starting.

The fix involves having the server look at the adjustable threshold.

Bug: 27505889
Change-Id: I5dbf686413e670dacbbecc9e0f838744e465f44f
Signed-off-by: Phil Burk <philburk@google.com>