Fix UniquePreferenceTest

Change-Id: I40f0c70dafe6992ad2a10a02419a6d1f54f96590
Fixes: 69917513
Test: atest
diff --git a/res/xml/zen_mode_settings.xml b/res/xml/zen_mode_settings.xml
index 82bef4d..dbd1d42 100644
--- a/res/xml/zen_mode_settings.xml
+++ b/res/xml/zen_mode_settings.xml
@@ -33,7 +33,8 @@
 
     <!-- Turn on DND button -->
     <!-- Layout preference doesn't obey allowDividerAbove, so put it in a PreferenceCategory -->
-    <PreferenceCategory>
+    <PreferenceCategory
+        android:key="zen_mode_settings_button_category">
         <com.android.settings.applications.LayoutPreference
             android:key="zen_mode_settings_button_container"
             android:selectable="false"