Mock BatterySaverStateMachine in PowerManagerServiceTests

Mock out BatterySaverStateMachine, so that PowerManagerServiceTests
don't depend on implementation of the class.
BatterySaverStateMachine lead to a crash on a device with over 1 day uptime,
because it called a method that couldn't be mocked on a BatterySaverController mock.

Test: atest com.android.server.power.PowerManagerServiceTest
Bug: 151828714
Change-Id: I6bdf0ae98f58b99f617ea3ae6addcc484aeb30a6