Muting ring volume slider disables notification

With volume_separate_notification flag enbaled, muting ring volume
slice will cause notification volume slice to gray out.

There used to be a bug in which notification slice would not get
updated in response to a change in ring volume mute/unmute broadcast.
The resulting erroneous behavior was notification slider would get to
zero but not get grayed out. To fix that bug, VolumeSliceHelper listens
to ring stream mute/unmute broadcasts and forwards them to notification
slice.

Bug: b/266072907
Test: make DEBUG_ROBOLECTRIC=1 ROBOTEST_FILTER="NotificationVolumePreferenceControllerTest|VolumeSliceHelperTest" RunSettingsRoboTests -j40

Change-Id: I2ab51f1272bf99a0c3d9ca285354052d00910c90