Add SystemBatteryConsumer.getPowerConsumedByApps()

Also, standardize the implementations of getConsumedPower()
to consistently return the total power consumed, not the
power unattributed to apps.

Bug: 183235836
Test: atest FrameworksCoreTests:com.android.internal.os.BatteryStatsTests
Change-Id: Iaedb29b9d7afd9843b0ab871e9b8656963db11ea