Make battery status refresh in SettingsActivity

Before this cl, the battery text in SettingsActivity is one time
update, won't refresh based on real battery status.

This cl elicits BatteryBroadcastReceiver from PowerUsageBase and
make it reusable in both SettingsActivity and battery page.

BatteryBroadcastReceiver will invoke callback if:
1. Battery level changed on integer level(100->99)
2. Battery status has changed(i.e. charging)

Bug: 29346753
Test: RunSettingsRoboTests

Change-Id: If522d15a700ccbc8bae24f5712e05ec27ea4cbfa

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

@@ -683,22 +683,28 @@ public class PowerUsageSummary extends PowerUsageBase {
     private static class SummaryProvider implements SummaryLoader.SummaryProvider {
         private final Context mContext;
         private final SummaryLoader mLoader;
+        private final BatteryBroadcastReceiver mBatteryBroadcastReceiver;
 
         private SummaryProvider(Context context, SummaryLoader loader) {
             mContext = context;
             mLoader = loader;
-        }
-
-        @Override
-        public void setListening(boolean listening) {
-            if (listening) {
-                // TODO: Listen.
+            mBatteryBroadcastReceiver = new BatteryBroadcastReceiver(mContext);
+            mBatteryBroadcastReceiver.setBatteryChangedListener(() -> {
                 BatteryInfo.getBatteryInfo(mContext, new BatteryInfo.Callback() {
                     @Override
                     public void onBatteryInfoLoaded(BatteryInfo info) {
                         mLoader.setSummary(SummaryProvider.this, info.chargeLabelString);
                     }
                 });
+            });
+        }
+
+        @Override
+        public void setListening(boolean listening) {
+            if (listening) {
+                mBatteryBroadcastReceiver.register();
+            } else {
+                mBatteryBroadcastReceiver.unRegister();
             }
         }
     }