title: how to animate one item of layer-list
body: <p>I have a layer list object, it contain two images, one is background,
  and the other is a rotation disk image which will be  raotated at the
  top of the background image. i.e. I use this layer-list as a linearlayout background,
  and I only want to animate "disk_bg" item of the layer-list;</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;layer-list xmlns:android="http://schemas.android.com/apk/res/
  android"&gt;
         &lt;item android:drawable="@drawable/player_bg" /&gt;
  
         &lt;item  android:top="166dp" &gt;
               &lt;bitmap  android:id="@+id/disk_bg" android:src="@drawable/cd"
         android:gravity="center" /&gt;
         &lt;/item&gt;
  </code></pre>
  
  <p></p>
  
  <p>I use this layer-list as a layout background, do you know how can I animate the disk_bg layer in my application?</p>
  
  <p>can you help me, many thanks to you~</p>
  
  <p>don't you get my question? or there is no way to do that?</p>