Fix settings crash when enhanced notification shown in search results

init NotificationBackend in constructor and set isSliceable to false in
search result since the NAS enabling logic is required and is in the
parent fragment ConfigureNotificationSettings.

Bug: 237251075
Test: test manually on device
Change-Id: I9082d6eda27784cf378a0d06304b5fc1e2ae6d7f
(cherry picked from commit 26ce9a98f0)

src/com/android/settings/notification/ConfigureNotificationSettings.java

@@ -107,7 +107,6 @@ public class ConfigureNotificationSettings extends DashboardFragment implements
         mNotificationAssistantPreferenceController =
                 use(NotificationAssistantPreferenceController.class);
         mNotificationAssistantPreferenceController.setFragment(this);
-        mNotificationAssistantPreferenceController.setBackend(new NotificationBackend());
     }
 
     private static List<AbstractPreferenceController> buildPreferenceControllers(Context context,

src/com/android/settings/notification/NotificationAssistantPreferenceController.java

@@ -44,6 +44,7 @@ public class NotificationAssistantPreferenceController extends TogglePreferenceC
     public NotificationAssistantPreferenceController(Context context) {
         super(context, KEY_NAS);
         mUserManager = UserManager.get(context);
+        mNotificationBackend = new NotificationBackend();
     }
 
     @Override
@@ -101,4 +102,9 @@ public class NotificationAssistantPreferenceController extends TogglePreferenceC
     void setBackend(NotificationBackend backend) {
         mNotificationBackend = backend;
     }
+
+    @Override
+    public boolean isSliceable() {
+        return (mFragment != null && mFragment instanceof ConfigureNotificationSettings);
+    }
 }