title: android:state_selected="true" doesn't work about ListView item
body: <p>I have a ListView which I want its item's background turn into green when the item is pressed and keep in grey when I release my finger.
  I use a selector(<strong>listview_item_indicator.xml</strong>) to achieve this:</p>
  
  <pre><code>&lt;selector xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;item android:drawable="@color/grey" android:state_selected="true" &gt;&lt;/item&gt;
      &lt;item android:drawable="@color/green" android:state_pressed="true"&gt;&lt;/item&gt;
  &lt;/selector&gt;
  </code></pre>
  
  <p>Then I update the xml file of the ListView item to apply this selecor like this:</p>
  
  <pre><code>android:background="@drawable/listview_item_indicator"
  </code></pre>
  
  <p>The result is it turn into green when I press the item,but it doesn't keep in grey when I release my finger(it turn to what it was before I pressed it).I think it means <code>&lt;item android:drawable="@color/green" android:state_pressed="true"&gt;&lt;/item&gt;</code> worked,but <code>&lt;item android:drawable="@color/grey" android:state_selected="true" &gt;&lt;/item&gt;</code> didn't work.Why?</p>