Add low power stats to batteryhistory

Exports SOC specific low power state info to batterystats service.
BatteryStats service queries the powerHAL module whenever and
updates the HistoryStepDetails whenever setBatteryState is called by the
BatteryService. The stats get appended to the battery history whenever
there is a change in the battery level.

Bug: 26564574
Change-Id: I02b4db00aec0c69d1558492224f3a4dc1c386fa6