Never store battery stats for cache

In PowerUsageBase, it will store battery stats if it has a configuration
change, which will make BatteryStatsLoaderHelper never get the correct
battery stats because it uses Bundle.EMPTY as the bundle message.

This cl:
1. Remove the store action even though it is configuration change.
2. Always use null to get battery stats
3. Always start a battery status check in register()

Bug: 63658232
Test: RunSettingsRoboTests
Change-Id: Ifbf970c63378ed66dddcdae4d952b7d1fd84216a