Punt battery status action when entering Settings main page

- Checking battery sharing states at the beging may have some performance issue, post battery status update to background thread to avoid ANR

Bug: 193600924
Test: make SettingsRoboTests
Change-Id: I3b73e6bd4f952bc832783b9b3b5d45db560d088c

tests/robotests/src/com/android/settings/fuelgauge/TopLevelBatteryPreferenceControllerTest.java

@@ -156,6 +156,7 @@ public class TopLevelBatteryPreferenceControllerTest {
 
     @Test
     public void getDashboardLabel_returnsCorrectLabel() {
+        mController.mPreference = new Preference(mContext);
         BatteryInfo info = new BatteryInfo();
         info.batteryPercentString = "3%";
         assertThat(mController.getDashboardLabel(mContext, info, true))