Make audio stop/pause non-blocking
Use audio_queue_post for audio_stop/audio_pause so they can be
called from any OS thread. audio_hard_stop now posts Q_AUDIO_STOP with
data=2; the audio handler treats data>=2 as a hard stop and frees
audiobuf_handle itself, avoiding cross-thread frees from callers.
Legacy data=1 path preserves the previous caller-frees behavior.