Master mute is valid when running fixed volume for automotive

In Automotive case, we set fixed volume to suggest no volume control
inside AudioFlinger and master mute is a mapping to the physical mute
button in a car, which toggles the global mute state.

Bug: 80474559
Test: adb shell input keyevent 164
Change-Id: I1d4fd143a98ee96f11850811c90e6d9932cab948