Fix mute/unmute sync issue with HDMI CEC

Full volume devices should not allow local mute changes. In the case of
HDMI CEC it is expected that all volume and mute changes are performed
on the HDMI sink.

By no longer allowing full volume devices to change their mute status
the sync issue between Android device and HDMI sink should be resolved.

BUG=145951160
Test: ADT-3 connected to TV supporting CEC volume changes.

Change-Id: I2b63493d86c2657408f7910f5a98b6b6eb412ad5