Add BatterySaver as SettingSlice

Convert BatterySaverController to TogglePrefController.
This was the first MasterSwitchController, so we needed
to add the preference case to updateState.

Test: Robotests
Change-Id: I32f3dee0b183807c66c19513b8b064af8f369c2a
Fixes: 67996992
6 files changed