Changed assist summary from "None" -> Empty

 - Updated summary so that search no longer displays none

Change-Id: I1c2e666f609dcae34f225f8f833d87b5cb20e74e
Fixes: 37160315
Test: Manual using settings app
diff --git a/res/xml/manage_assist.xml b/res/xml/manage_assist.xml
index a96fb6b..120309a 100644
--- a/res/xml/manage_assist.xml
+++ b/res/xml/manage_assist.xml
@@ -22,7 +22,7 @@
     <com.android.settings.widget.GearPreference
         android:key="default_assist"
         android:title="@string/default_assist_title"
-        android:summary="@string/app_list_preference_none"
+        android:summary="@string/summary_placeholder"
         android:fragment="com.android.settings.applications.assist.DefaultAssistPicker"/>
 
     <Preference