Begin transition from BatteryStatsHelper to BatteryUsageStats API.

For now, keep both BatteryStatsHelper and BatteryUsageStats in play.
The plan is to transition from the former to the latter, one usage
at a time.  When all is said and done, all references to
BatteryStatsHelper will be gone.

Bug: 173745486
Test: atest --host SettingsRoboTests

Change-Id: I37e1dfff0043b1845992f18d72067bb547bb69ff