Make some notification related slices public.

Only PreferenceControllers with isPublicSlice() set to true are exposed
to other apps. Others will be Settings only.

Bug: 141088937
Test: robotests
Change-Id: I6e7b9d45084ea9944143e606bfac56b2cae3e8a3

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

@@ -44,6 +44,11 @@ public class NotificationVolumePreferenceController extends
         return TextUtils.equals(getPreferenceKey(), KEY_NOTIFICATION_VOLUME);
     }
 
+    @Override
+    public boolean isPublicSlice() {
+        return true;
+    }
+
     @Override
     public String getPreferenceKey() {
         return KEY_NOTIFICATION_VOLUME;