Initial split QWERTY layout

Change [2/3]

Add an attribute that's used in layout xmls for specifying
the split-keyboard layout.
This attribute will be checked/set by the KeyboardBuilder which knows about
the "splittability" of the keyboard.

Bug: 4968173
Change-Id: I5d792e570b9648e09022e9b7e92a972a161009ee
diff --git a/java/res/values/attrs.xml b/java/res/values/attrs.xml
index c5c6326..8ee859b 100644
--- a/java/res/values/attrs.xml
+++ b/java/res/values/attrs.xml
@@ -483,6 +483,8 @@
         <attr name="localeCode" format="string" />
         <attr name="languageCode" format="string" />
         <attr name="countryCode" format="string" />
+        <!-- Enable split keyboard layout. Disabled by default. -->
+        <attr name="isSplitLayout" format="boolean" />
     </declare-styleable>
 
     <declare-styleable name="Keyboard_KeyStyle">