title: Animate the stroke width of a Shape
body: <p>I have a thick circle with a transparent centre:</p>
  
  <pre><code>&lt;shape
      xmlns:android="http://schemas.android.com/apk/res/android"
      android:shape="oval"&gt;
      &lt;solid
          android:color="@android:color/transparent"/&gt;
      &lt;stroke
          android:width="24dp"
          android:color="@android:color/white"/&gt;
      &lt;size
          android:width="72dp"
          android:height="72dp"/&gt;
  &lt;/shape&gt;
  </code></pre>
  
  <p>and would like to animate a reduction in the stroke value so the transparent center dilates, like an iris.</p>
  
  <pre><code>&lt;set xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;objectAnimator
          android:propertyName="????"
          android:valueFrom="24dp"
          android:valueTo="4dp"
          android:duration="750"
          /&gt;
  &lt;/set&gt;
  </code></pre>
  
  <p>... but I don't know what to specify as the property name. "strokeWidth" doesn't seem to work. I'm not even sure how to pull it off programatically because GradientDrawable.setStroke() requires both a width and a color and objectAnimator can only manipulate single parameter properties.</p>