Create loader for Power HAL services

Move logic to load underlying Power HAL services to a static class in
services/powermanager, with tests. This can now be reused by
BatteryStatsService and PowerManagerService, removing the dependency
between them.

Bug: 150878220
Test: atest powermanager_test
Change-Id: I52eec130811d49111526f3e152faf4302518c5a8