BatteryStats: Update to use power.stats HAL

This CL updates BatteryStatsService to use power.stats HAL if available.
power.stats HAL is the new HAL that will be collecting platform and
subsystem power statistics going forward. BatteryStatsService will still
fall back to using the existing power HAL if power.stats HAL is not
implemented.

Bug: 120552155
Test: Disconnected the device from the charger, let battery drain 1-2%,
then ran "adb shell dumpsys batterystats" and visually inspected.
Test: Did the above test with and without power.stats HAL available
Test: Took a bugreport and verified that subsystem stats show up in
Volta Historian.

Change-Id: I5eb63fcbbf87a134bd8c34a11330db8b55df3894