Introduce overlaid BatterySaverGoogleScreen

Test: atest BatterySaverScreenTest
Bug: 368359126
Flag: com.android.settings.flags.catalyst_battery_saver_screen
Change-Id: Iac80ea417564fd3c6a1859269109a77e08b0815f

src/com/android/settings/fuelgauge/batterysaver/BatterySaverScreen.kt

@@ -23,7 +23,7 @@ import com.android.settingslib.metadata.preferenceHierarchy
 import com.android.settingslib.preference.PreferenceScreenCreator
 
 @ProvidePreferenceScreen
-class BatterySaverScreen : PreferenceScreenCreator {
+open class BatterySaverScreen : PreferenceScreenCreator {
     override val key: String
         get() = KEY
 
@@ -39,9 +39,8 @@ class BatterySaverScreen : PreferenceScreenCreator {
 
     override fun hasCompleteHierarchy() = false
 
-    override fun getPreferenceHierarchy(context: Context) = preferenceHierarchy(this) {
-        +BatterySaverPreference()
-    }
+    override fun getPreferenceHierarchy(context: Context) =
+        preferenceHierarchy(this) { +BatterySaverPreference() order -100 }
 
     companion object {
         const val KEY = "battery_saver_screen"