Sticky battery saver

- When battery saver is enabled manually (i.e. via PM.setPowerSaveMode()),
it'll stick, and we'll re-enable battery saver even after a reboot
or a charge.

- Extracted all battery saver state transition logic into a separate
class.

Fix: 75033216
Bug: 74120126
Test: Manual test with "dumpsys battery set ...."
Test: atest $ANDROID_BUILD_TOP/frameworks/base/services/tests/servicestests/src/com/android/server/power/batterysaver/BatterySaverStateMachineTest.java
Change-Id: If020cd48f341b339783fe09dd35bc7199e737a52
Test: dumpsys power
Test: incident_report power
Test: atest CtsBatterySavingTestCases