Read flag to show/hide notification slider

Replace the build-time boolean resource with a run time device config flag that
signals whether the ring notification slider should be displayed or not

Bug: b/38477228
Test: make DEBUG_ROBOLECTRIC=1 ROBOTEST_FILTER=NotificationVolumePreferenceControllerTest RunSettingsRoboTests -j40
      make DEBUG_ROBOLECTRIC=1 ROBOTEST_FILTER=RingVolumePreferenceControllerTest RunSettingsRoboTests -j40
Change-Id: I8b9a2cbd5af7fa1bba56ff9ba62771d677d4a932