Remove dependency on PowerManagerService from BatteryStatsService

Use helper class PowerHalLoader from native/powermanager/services to
load Power HALs v1.0 and 1.1.

Refactor: add 'Locked' sufix and mutex comment to every method from
BatteryStatsService that implicitly requires a lock on the mutex to be
held from the calling site.

Bug: 150878220
Test: atest powermanager_test
Change-Id: I46b6a24511ed26787758f2bb8f2cab0b2da31e67