Fix races related to volume and mute

Fix race conditions when setting master volume, master mute, stream
volume, stream mute for a playback thread, and when reading stream
volume of a playback thread.  Lock order is AudioFlinger, then thread.

Rename streamVolumeInternal to streamVolume_l, comment, and use it to
implement streamVolume().

Code size reduction:
 - Remove dead code: AudioFlinger::PlaybackThread::masterVolume, masterMute, streamMute.
 - Change return type of non-binder methods that always succeed from status_t to void.
 - Remove virtual from volume and mute methods that don't need it.

This change saves 228 bytes but decreases performance of binder operations
due to the added locks.

Change-Id: Iac75abc1f54784873a667d1981b2e08f8f31e5c9