Enhance battery saver event log

- Add reason to on/off log
- Add "threshold changed" event log

Bug: 78833408
Test: Manual test + logcat
Test:   atest $ANDROID_BUILD_TOP/frameworks/base/services/tests/servicestests/src/com/android/server/power/batterysaver/BatterySaverStateMachineTest.java
Change-Id: Iddaff8dfa585923c3581552200ea2734b2c1a16d