title: Error when trying to change the corner radius of Android ProgressBar
body: <p>I have a <code>ProgressBar</code> widget with <code>Widget.AppCompat.ProgressBar.Horizontal</code> style:</p>
  <pre><code>&lt;ProgressBar
      style=&quot;@style/Widget.AppCompat.ProgressBar.Horizontal&quot;
      android:layout_width=&quot;match_parent&quot;
      android:layout_height=&quot;wrap_content&quot;/&gt;
  </code></pre>
  <p>When digging into <code>Widget.AppCompat.ProgressBar.Horizontal</code> (it uses <code>Widget.Material</code> internally) style definition in the AppCompat source codes, I've found that for API 21+ it uses a following drawable:</p>
  <pre><code>&lt;style name=&quot;Widget.Material.ProgressBar.Horizontal&quot; parent=&quot;Widget.ProgressBar.Horizontal&quot;&gt;
      &lt;item name=&quot;progressDrawable&quot;&gt;@drawable/progress_horizontal_material&lt;/item&gt;
      ...
  &lt;/style&gt;
  </code></pre>
  <p>Here is a fragment of <code>progress_indeterminate_horizontal_material</code>:</p>
  <pre><code>&lt;layer-list xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;&gt;
      &lt;item android:id=&quot;@id/background&quot;
            android:gravity=&quot;center_vertical|fill_horizontal&quot;&gt;
          &lt;shape android:shape=&quot;rectangle&quot;
                 android:tint=&quot;?attr/colorProgressBackgroundNormal&quot;&gt;
              &lt;corners android:radius=&quot;?attr/progressBarCornerRadius&quot; /&gt;
              &lt;size android:height=&quot;@dimen/progress_bar_height_material&quot; /&gt;
              &lt;solid android:color=&quot;@color/white_disabled_material&quot; /&gt;
          &lt;/shape&gt;
      &lt;/item&gt;
  </code></pre>
  <p>It uses <code>?attr/colorProgressBackgroundNormal</code> attribute, however when I'm trying to set the value of this attribute in my application theme with the following line:</p>
  <pre><code>&lt;item name=&quot;progressBarCornerRadius&quot;&gt;2dp&lt;/item&gt;
  </code></pre>
  <p>then I'm getting a compilation error:</p>
  <pre><code>AAPT: error: style attribute 'attr/progressBarCornerRadius (aka com.application:attr/progressBarCornerRadius)' not found.
  </code></pre>