AudioFlinger: synchronize OutputTracks start on duplicating threads

When playback starts on a duplicating thread and target output
mixer threads exit standby, it is possible that one output stream takes
a long time to actually start playback (e.g. for Bluetooth software
encoder path). In this case, the OutputTrack on the other mixer thread
will underrun and audio will get choppy on this output stream.

In order to avoid this, an actual stream start detection mechanism is
added on the mixer thread and the OutputTrack write method will wait
for the target thread to actually start when exiting standby before
sending actual audio.

Bug: 218803475
Test: repro steps in bug
Change-Id: I0eec8c5817c97f7b7643a7151037aff5dc73f3ae
Merged-In: I0eec8c5817c97f7b7643a7151037aff5dc73f3ae