title: Android Selector Drawable doesnt work with attributes
body: <p>I am using attr to create a selector drawable for my project so that once i change theme colors, i dont have to make any change in the drawable file. I am using following libs:</p>
  
  <pre><code>compile 'com.android.support:appcompat-v7:+'
  compile 'com.android.support:cardview-v7:+'
  compile 'com.android.support:design:22.2.0'
  </code></pre>
  
  <p>Here is the source code for drawable:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;selector xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;item android:drawable="?attr/colorPrimary" android:state_enabled="true" android:state_window_focused="false"/&gt;
      &lt;item android:drawable="?attr/colorPrimaryDark" android:state_pressed="true"/&gt;
      &lt;item android:drawable="@android:color/darker_gray" android:state_enabled="false"/&gt;
      &lt;item android:drawable="?attr/colorPrimary"/&gt;
  &lt;/selector&gt;
  </code></pre>
  
  <p>in this same code, if i replace attributes with colors defined in colors.xml file, the same drawable works.</p>
  
  <p>Sample drawable with colors:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;selector xmlns:android="http://schemas.android.com/apk/res/android"&gt;
  &lt;item android:drawable="@color/color_primary" android:state_enabled="true" android:state_window_focused="false"/&gt;
      &lt;item android:drawable="@color/color_primary_dark" android:state_pressed="true"/&gt;
      &lt;item android:drawable="@android:color/darker_gray" android:state_enabled="false"/&gt;
      &lt;item android:drawable="@color/color_primary"/&gt;
  &lt;/selector&gt;
  </code></pre>
  
  <p>Thanks in advance!</p>