ThemePicker: Define clocks stub package

There doesn't actually need to be anything in the stub; it's just used
as a check for whether clock theming is allowed.

Change-Id: Ie1f1302b512fe6b0799cf3837a1f18446d0651ee

overlay/common/packages/apps/ThemePicker/res/values/override.xml

@@ -17,6 +17,7 @@
 -->
 <resources>
     <string name="themes_stub_package" translatable="false">org.lineageos.customization</string>
+    <string name="clocks_stub_package" translatable="false">org.lineageos.customization</string>
 
     <!-- List of packages whose icons are used to preview the icon shape for a theme. These are
         typically GMS apps so they should be available in GMS devices. -->