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

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

@@ -43,6 +43,7 @@ public class NotificationAssistantPreferenceController extends TogglePreferenceC
     public NotificationAssistantPreferenceController(Context context) {
         super(context, KEY_NAS);
         mUserManager = UserManager.get(context);
+        mNotificationBackend = new NotificationBackend();
     }
 
     @Override
@@ -100,4 +101,9 @@ public class NotificationAssistantPreferenceController extends TogglePreferenceC
     void setBackend(NotificationBackend backend) {
         mNotificationBackend = backend;
     }
+
+    @Override
+    public boolean isSliceable() {
+        return (mFragment != null && mFragment instanceof ConfigureNotificationSettings);
+    }
 }