Extract the not-strictly-modes-related parts of ZenModesBackend

So that when it's moved to SettingsLib, it doesn't need to carry that baggage.

Bug: 346519570
Test: atest com.android.settings.notification.modes
Flag: android.app.modes_ui
Change-Id: I7911a521d96f5dbac2c2395171d324b7b54b8b07

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

@@ -41,6 +41,7 @@ abstract class ZenModesFragmentBase extends RestrictedDashboardFragment {
     protected Context mContext;
 
     protected ZenModesBackend mBackend;
+    protected ZenHelperBackend mHelperBackend;
 
     // Individual pages must implement this method based on what they should do when
     // the device's zen mode state changes.
@@ -59,6 +60,7 @@ abstract class ZenModesFragmentBase extends RestrictedDashboardFragment {
     public void onAttach(@NonNull Context context) {
         mContext = context;
         mBackend = ZenModesBackend.getInstance(context);
+        mHelperBackend = ZenHelperBackend.getInstance(context);
         super.onAttach(context);
         mSettingsObserver.register();
     }