aaudio: fix STOP hang by unlocking around join()

Unlock and lock around the join() because the callback
thread that we are joining may be blocked by the lock.
Check for DISCONNECTED state so the error callback is
also joined correctly.
Remove unnecessary waitForStateChange that could cause
a timeout delay on DISCONNECT.

Bug: 134963902
Test: test_stop_hang (see bug report)
Change-Id: I5080fa655da35b3221234a2bcafb6ccd71d3ca27