Move optimization mode backup timing into onPause

- Move to onPause to avoid some corner case (like kill apps from recent
 app won't trigger onDestroy)

Bug: 195306545
Test: make SettingsRoboTests
Change-Id: I89445a7138c30e6c869ce1c5c9a3f818997d99a7

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

@@ -856,7 +856,7 @@ public class AdvancedPowerUsageDetailTest {
                 .thenReturn(BatteryOptimizeUtils.MODE_UNRESTRICTED);
         mFragment.mEnableTriState = false;
 
-        mFragment.notifyBackupManager();
+        mFragment.onPause();
 
         verifyZeroInteractions(mBackupManager);
     }