Propagate mute event to both ring volume slices

Fix a bug where introduction of a separate_ring_volume slice meant that
the original ring_volume slice would not get notified of ring stream
muting event.

Bug: b/266855922

Test: make DEBUG_ROBOLECTRIC=1 ROBOTEST_FILTER=VolumeSliceHelperTest RunSettingsRoboTests -j40
Change-Id: Ifb5ebe4e7d9bde3d14336891cce3fbee167a5911