Add channel mask in AudioSink

Add support for specifying a channel mask when opening an AudioSink.
  This parameter does not replace the channel count parameter in order
  to not have to duplicate the logic to derive a mask from the
  channel count everywhere an AudioSink is used without a known mask.

A mask of 0 (CHANNEL_MASK_USE_CHANNEL_ORDER) means a mask will
  be automatically derived from the number of channels.

Update existing AudioSink implementations to use the channel mask,
  and users of AudioSink to specify the mask if available, and
  CHANNEL_MASK_USE_CHANNEL_ORDER otherwise.

Change-Id: Ifa9bd259874816dbc25ead2b03ea52e873cff474