Fix notification assistant setting

Test: make cts-verifier
Change-Id: I9029e792ef3d32e433012d86ba828e3c741799cb
diff --git a/res/xml/app_default_settings.xml b/res/xml/app_default_settings.xml
index 27200e7..1733bab 100644
--- a/res/xml/app_default_settings.xml
+++ b/res/xml/app_default_settings.xml
@@ -72,6 +72,7 @@
     <Preference
         android:key="default_notification_asst_app"
         android:title="@string/default_notification_assistant"
+        android:fragment="com.android.settings.applications.defaultapps.DefaultNotificationAssistantPicker"
         android:order="-13"/>
 
     <Preference