Add mode: Support for app-provided modes

(This completes the add-mode flow except for the choose-a-name-and-icon step for custom modes).

Bug: 326442408
Flag: android.app.modes_ui
Test: atest com.android.settings.notification.modes
Change-Id: I7aceec01ed54d804bcac53d932277c243c1f81bf

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

@@ -26,6 +26,8 @@ import android.os.UserManager;
 import android.provider.Settings.Global;
 import android.util.Log;
 
+import androidx.annotation.VisibleForTesting;
+
 import com.android.settings.dashboard.RestrictedDashboardFragment;
 import com.android.settingslib.notification.modes.ZenModesBackend;
 
@@ -57,6 +59,11 @@ abstract class ZenModesFragmentBase extends RestrictedDashboardFragment {
         return TAG;
     }
 
+    @VisibleForTesting(otherwise = VisibleForTesting.NONE)
+    void setBackend(ZenModesBackend backend) {
+        mBackend = backend;
+    }
+
     @Override
     public void onAttach(@NonNull Context context) {
         mContext = context;