title: Format attribute value "android:drawable" not valid
body: <p>I'm trying to create custom attributes to my button but I dont know which format I must use to images in attributes declaration...</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;resources&gt;
  
      &lt;declare-styleable name="TCButton"&gt;
          &lt;attr name="Text" format="string"/&gt;
          &lt;attr name="BackgroundImage" format="android:drawable"  /&gt;
      &lt;/declare-styleable&gt;
  
  
  &lt;/resources&gt;
  </code></pre>
  
  <p>Error is in the format="android:drawable"...</p>