Calls to ALSA handles must be serialized by the caller.

Change-Id: Ia62a1b6867cb67b9a48f7acf87ead02ba0ea040c
Fix up volume control unit tests to handle case where full range of
volume type is used (i.e. there are no invalid volumes).

Change-Id: If95eb358458f957f5c3fc4e42d323acc029f7325
Fix SinkPlayer::GetSinkCount to return count of sinks instead of
true/false.

Change-Id: I0e0cff0f78b3c1381d495c2f7013eda3222ce24f