AudioTrack: Prevent server from reading client data after stop

Prior to this CL, if an AudioTrack client wrote audio data after
AudioTrack stop(), but before the track was drained by the server,
the newly written client data would be consumed in the drain.

We now limit the server read to the client write position on stop.
This interlocking is essential for rapid asynchronous AudioTrack
command processing.

Test: AudioTrack CTS, SoundPool looping, bug test case
Bug: 75788313
Change-Id: Ib70e3dc46afe047a8c6cf1fb906a618b3c66cc7f