Convert BatterySaverButton controller to Slice compatible.

- Update preference key to match the key defined in SettingsSlicesContract
- Model TwoStateButtonPreference similar to TwoStatePreference (add
  setChecked, isChecked method)
- Remove TwoStateButtonPreferenceController entirely because all methods
  are moved into Preference directly for better encapsulation.
- Make BatterySaverButtonPrefController direclty implement
  TogglePreferenceController. It was not possible before because the
  interface between TwoStateButtonPreferene is too different from
  TwoStatePreference.

Bug: 80106671
Test: robotests
Change-Id: Ib72807dcf1b36e959e08df8d80538c3f9f79b76d