Refactor battery saver logic + add "per device" setting

- Extract the battery saver mode transition logic to BatterySaverController.

This now also supports running different code when screen turns on and off.

- BatterySaverPolicy now takes a "per-device configuration" from config.xml,
which can be overwritten via a global setting. We'll use this to set up
max CPU frequencies.

- The actual part to write max CPU frequencies is not finished yet.

Test: atest BatterySaverPolicyTest
Bug: 68769804
Change-Id: Ife38c2cd94ac9902911b005dbbca8b0d0a62e6d7