audio: Fix unexpected mute on full volume device

Issue:
Full volume devices should not be mutable as their define. However,
when AudioManager#setStreamVolume is called with index zero while
using a full volume device, audio is muted unexpectedly. This creates
inconsistency with AudioManager#adjustStreamVolume behavior, which
correctly prevents muting on full volume devices.

Root Cause:
AudioService incorrectly applies mute state in onSetStreamVolume
even for full volume devices.

Solution:
Prevent mute state changes when setStreamVolume is called for
full volume devices.

Bug: 426346130
Test: manual - adb shell cmd media_session volume --set 0
      while using full volume device.
Flag: EXEMPT bugfix

Change-Id: I9fd99fd92b96bad49551ff9bf780fdcd7f28a72b
Signed-off-by: Tomoki Yonezawa <Tomoki.Yonezawa@sony.com>