title: Android - Using color attr in pre-lollipop drawables causes exception
body: <p>I'm going to use a color attr in my drawable but it only works on API 21+ and crashes on API 18. How to fix this issue?</p>
  
  <pre><code>&lt;style name="EasyIntro.Light" parent="Theme.AppCompat.Light"&gt;
          &lt;item name="toggleIndicatorsEnabled"&gt;@color/toggleIndicatorsEnabledColor&lt;/item&gt;
      &lt;/style&gt;
  
      &lt;color name="toggleIndicatorsEnabledColor"&gt;#7f000000&lt;/color&gt;
  
      &lt;attr name="toggleIndicatorsEnabled" format="color|reference"/&gt;
  
      &lt;item
              android:state_enabled="true"
              android:state_pressed="true"&gt;
              &lt;shape android:shape="oval"&gt;
                  &lt;solid android:color="?attr/toggleIndicatorsEnabled"/&gt;
              &lt;/shape&gt;
          &lt;/item&gt;
  </code></pre>