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

src/com/android/settings/notification/RingerModeAffectedVolumePreferenceController.java

@@ -140,11 +140,18 @@ public abstract class RingerModeAffectedVolumePreferenceController extends
         return valueUpdated;
     }
 
-    protected void updateRingerMode() {
+    /**
+     * Updates UI Icon in response to ringer mode changes.
+     * @return whether the ringer mode has changed.
+     */
+    protected boolean updateRingerMode() {
         final int ringerMode = mHelper.getRingerModeInternal();
-        if (mRingerMode == ringerMode) return;
+        if (mRingerMode == ringerMode) {
+            return false;
+        }
         mRingerMode = ringerMode;
         selectPreferenceIconState();
+        return true;
     }
 
     /**