Fix issue 2115450: a2dp thread is started, even though we are only connected to heads...

Fix issue 2115450: a2dp thread is started, even though we are only connected to headset and not playing music.

This is due to a regression introduced by change 24114: when no audio tracks are ready for mixing, 0s are written to audio hardware. However this should only happen if tracks have already been mixed since the audio flinger thread woke up.
Also do not write 0s to audio hardware in direct output threads when audio format is not linear PCM.