audio: Fix race condition in AudioTrack underrun.

When audio flinger mixer removes an AudioTrack from the
active list in case of underrun, it is possible that the
client has written a full buffer just after the underrun detection and
is blocked waiting for more space to write. In this case, the client
will never detect the DISABLED flag and the track never be restarted.

Also implement missing DISABLE flag detection in server side audio tracks
(OutputTrack and PatchTrack).

bug: 27567768
Change-Id: I8d0753429d4113498258b1f61bd8ac5939a612f0