aaudio: check for callback thread outside the lock

Calling AAudioStream_requestStop() from a callback is illegal.
But it should just return and error and not hang.
The hang can occur if AAudioStream_close() is called at the same time
from another thread.

Bug: 184370309
Test: adb shell test_disconnect_race
Change-Id: I5e0752611d4d3d3dd994a10e82a339fa39e65d42