Use BatterySaverReceiver in battery saver settings

Use this receiver to listen to following updates:
1. Plug state: if it is plugged in, disable this two state
button, otherwise make it enabled
2. low power mode: If mode on, show "turn off" otherwise show
"turn on"

Bug: 72228477
Test: RunSettingsRoboTests
Change-Id: Ib3d3df9f9a5fbac8b1b876c9e193beb75f522ebd

src/com/android/settings/fuelgauge/BatterySaverReceiver.java

@@ -69,7 +69,7 @@ public class BatterySaverReceiver extends BroadcastReceiver {
         mBatterySaverListener = lsn;
     }
 
-    interface BatterySaverListener {
+    public interface BatterySaverListener {
         void onPowerSaveModeChanged();
         void onBatteryChanged(boolean pluggedIn);
     }