title: Data Binding class not generated
body: <p>I am using Data Binding in my project, when using <code>&lt;layout&gt;</code> and <code>&lt;data&gt;</code> in my xml binding class is not generated.</p>
  
  <p>For example i have activity_main.xml </p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;layout xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;data&gt;    &lt;/data&gt;
      &lt;RelativeLayout
          android:layout_width="match_parent"
          android:layout_height="match_parent"&gt;
      &lt;/RelativeLayout&gt;
  &lt;/layout&gt;
  </code></pre>
  
  <p>Now if i am writing <code>ActivityMainBinding</code> in my activity/fragment it shows error that class is not available. But after including <code>&lt;variable&gt;</code> in my xml file, it is able to generate <code>ActivityMainBinding</code> class.</p>
  
  <p>Android Studio : <strong>2.1.3</strong><br>
  Classpath : <strong>com.android.tools.build:gradle:2.1.3</strong><br>
  minSdkVersion <strong>16</strong><br>
  targetSdkVersion <strong>24</strong><br>
  buildToolsVersion <strong>24.0.0</strong></p>