Add null check when restart batteryinfo loader

Even though we unregister contentObserver in onPause and register
in onResume, callback still be called while fragment is detached.

Anyhow add a null check in settings to stop crash

Fixes: 131905853
Test: RunSettingsRoboTests
Change-Id: I8c0c2c04c3b8d942e0c97cf71a7d3e735a24b467