Before this cl, the battery text in SettingsActivity is one time
update, won't refresh based on real battery status.
This cl elicits BatteryBroadcastReceiver from PowerUsageBase and
make it reusable in both SettingsActivity and battery page.
BatteryBroadcastReceiver will invoke callback if:
1. Battery level changed on integer level(100->99)
2. Battery status has changed(i.e. charging)
Bug: 29346753
Test: RunSettingsRoboTests
Change-Id: If522d15a700ccbc8bae24f5712e05ec27ea4cbfa
