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

res/xml/battery_saver_settings.xml

@@ -18,32 +18,35 @@
     xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:settings="http://schemas.android.com/apk/res-auto"
     android:title="@string/battery_saver"
-    android:key="battery_saver">
+    android:key="battery_saver_page">
 
     <!-- Turn on automatically -->
     <SwitchPreference
         android:key="auto_battery_saver"
         android:title="@string/battery_saver_auto_title"
-        settings:controller="com.android.settings.fuelgauge.batterysaver.AutoBatterySaverPreferenceController"/>
+        settings:controller="com.android.settings.fuelgauge.batterysaver.AutoBatterySaverPreferenceController" />
 
     <com.android.settings.widget.SeekBarPreference
         android:key="battery_saver_seek_bar"
         android:title="@string/battery_saver_seekbar_title_placeholder"
         android:max="75"
-        android:min="5"/>
+        android:min="5" />
 
     <com.android.settings.widget.TwoStateButtonPreference
-        android:key="battery_saver_button_container"
+        android:key="battery_saver"
+        android:title="@string/battery_saver"
         android:selectable="false"
         settings:textOn="@string/battery_saver_button_turn_on"
-        settings:textOff="@string/battery_saver_button_turn_off"/>
+        settings:textOff="@string/battery_saver_button_turn_off"
+        settings:platform_slice="true"
+        settings:controller="com.android.settings.fuelgauge.batterysaver.BatterySaverButtonPreferenceController" />
 
     <PreferenceCategory
         android:key="battery_saver_footer">
         <com.android.settingslib.widget.FooterPreference
             android:key="battery_saver_footer_preference"
             android:title="@*android:string/battery_saver_description"
-            android:selectable="false"/>
+            android:selectable="false" />
     </PreferenceCategory>
 
 </PreferenceScreen>