Add keyboardLayoutSet attribute to <case> condition

Change-Id: I017c4a3c1c7b2e4c6f607940078f61e120042270
diff --git a/java/res/values/attrs.xml b/java/res/values/attrs.xml
index 589adec..631c35d 100644
--- a/java/res/values/attrs.xml
+++ b/java/res/values/attrs.xml
@@ -379,6 +379,7 @@
     </declare-styleable>
 
     <declare-styleable name="Keyboard_Case">
+        <attr name="keyboardLayoutSet" format="string" />
         <!-- This should be aligned with KeyboardLayoutSet_Element's elementName. -->
         <attr name="keyboardLayoutSetElement" format="enum|string">
             <enum name="alphabet" value="0" />