title: Re-use xml gradient in other xml drawable
body: <p>I have a gradient defined in <code>bg_gradient.xml</code>:</p>
  
  <pre><code>&lt;shape xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;gradient
          android:startColor="@color/colorPrimaryDark"
          android:endColor="@color/colorPrimary"
          android:type="linear"
          android:angle="315"/&gt;
  &lt;/shape&gt;
  </code></pre>
  
  <p>now I'd like to create a drawable xml which is supposed to be a circle filled with this gradient.
  This is what I have so far:</p>
  
  <pre><code>&lt;shape xmlns:android="http://schemas.android.com/apk/res/android"
      android:shape="oval"&gt;
      &lt;size android:height="@dimen/size_update_indicator"
          android:width="@dimen/size_update_indicator"&gt;
      &lt;/size&gt;
      &lt;gradient /* somehow I need to refer to bg_gradient here*/ ... /&gt;
  &lt;/shape&gt;
  </code></pre>
  
  <p>Is there a way I can refer to the existing gradient instead of just copy pasting it?</p>