Add "enabled" as an attribute of Key in Keyboard xml

If the key is marked as "enabled=false", the key will not respond to be pressed.

Bug: 3384942
Change-Id: I1ea82d9f3dfa5a9916ff3bbc736ea7e6c77426ae
diff --git a/java/res/values/attrs.xml b/java/res/values/attrs.xml
index 9759e0e..b6d5fdc 100644
--- a/java/res/values/attrs.xml
+++ b/java/res/values/attrs.xml
@@ -140,6 +140,8 @@
         <attr name="keyStyle" format="string" />
         <!-- Shift key icon for shifted state -->
         <attr name="shiftedIcon" format="reference" />
+        <!-- The key is enabled and responds on press. -->
+        <attr name="enabled" format="boolean" />
     </declare-styleable>
 
     <declare-styleable name="Keyboard_Row">