[SB][Battery] Log when power save changes come in.

This will help us debug issues like b/334410974 when it seems like SysUI
didn't update power save correctly.

Bug: 334410974
Test: `adb shell dumpsys activity service
com.android.systemui/.SystemUIService BatteryControllerLog` -> verify
power save logs show
Flag: NONE (just adding logging)

Change-Id: Ia195104e47cd931f685ffc2f89a3a6facc06a4c2