Aggregate power data in AggregateBatteryConsumers

as opposed to SystemBatteryConsumers, which will be removed
in a follow-up CL

Bug: 186006876
Test: atest FrameworksCoreTests:com.android.internal.os.BatteryStatsTests

Change-Id: Ie4166217be211d92293c2a107762801a7d3c206f