Refine battery settings layout

- Add loading text to progress bar to prevent UI flaky
 - Setup enable/disable state for battery saver switch
 - Remove battery saver page divider

Bug: 177964551
Bug: 177964753
Bug: 184626649
Test: make SettingsRoboTests, visually
Change-Id: I8e05c0c0f02710729e1f59eff23fb3f425770de9

src/com/android/settings/fuelgauge/batterysaver/BatterySaverButtonPreferenceController.java

@@ -122,5 +122,8 @@ public class BatterySaverButtonPreferenceController extends
 
     @Override
     public void onBatteryChanged(boolean pluggedIn) {
+        if (mPreference != null) {
+            mPreference.setEnabled(!pluggedIn);
+        }
     }
 }