Move ZenModesBackend to SettingsLib.

This will allow us to access it from SystemUI.

Bug: 346519570
Test: builds
Flag: EXEMPT trivial refactor
Change-Id: I5bc480bd4eb0cbf8a26989dd11c064e66e5ee70e

src/com/android/settings/notification/modes/ZenModeFragmentBase.java

@@ -29,6 +29,7 @@ import androidx.preference.PreferenceScreen;
 
 import com.android.settings.R;
 import com.android.settingslib.core.AbstractPreferenceController;
+import com.android.settingslib.notification.modes.ZenMode;
 
 import java.util.List;