Unmute when turning system audio mode on.

ag/5428480

This is useful for devices that disables muting.
When TV sends out muting to the device but the device later turns system
audio mode back on, it should still unmute itself.

Disabling muting should only disable self mute. But still support
unmute.

Test: manual
Bug: 118890232
Change-Id: Ief4435c786262a1981a29627b837ef0a4831157a