title: remove default radio button checkbox when using a custom selector
body: <p>I am attempting to style a radio button and so I created a selector.  Everything works except the standard radio button image still shows up.  How do I remove the default radio button checkbox.  I tried assigning the drawable to an empty string, but the compiler does not like it.</p>
  
  <p>Here is the code of one of my selector states:</p>
  
  <pre><code>&lt;item android:state_checked="true" android:drawable=""&gt;
      &lt;shape&gt;
          &lt;gradient
              android:startColor="#808080"
              android:centerColor="#2F2F2F"
              android:endColor="#2F2F2F"
              android:angle="270" /&gt;
          &lt;stroke
              android:width="2dp"
              android:color="@color/black" /&gt;
          &lt;corners
              android:radius="5dp" /&gt;
      &lt;/shape&gt;
  &lt;/item&gt;
  </code></pre>