Keyboard XML file supports include and merge tag

Keyboard XML file can include other keyboard XML file using directive
<include keyboardLayout="@xml/...">.  The keyboard XML file which is
included must have <merge> tag as root element.

Change-Id: I06c35fe7b3db5232acdb33f73a79f38d31261b32
diff --git a/java/res/values/attrs.xml b/java/res/values/attrs.xml
index 1900214..318286e 100644
--- a/java/res/values/attrs.xml
+++ b/java/res/values/attrs.xml
@@ -129,4 +129,8 @@
              requested keyboard mode, the row will be skipped. -->
         <attr name="keyboardMode" format="reference" />
     </declare-styleable>
+
+    <declare-styleable name="BaseKeyboard_Include">
+        <attr name="keyboardLayout" format="reference" />
+    </declare-styleable>
 </resources>