AudioService: handle stream mute by stream alias

Handle stream mute/unmute requests at the stream
alias level (like is done volume changes) to be consistent.
E.g a request to mute/unmute a stream will affect all streams
sharing the same alias for volume management.

Bug: 18844550.
Change-Id: Ie2e463c58df0371589150f0ebb9b36c38b14e1be