title: Show up/back button on android nested PreferenceScreen?
body: <p>I have a two level PreferenceScreen:</p>
  
  <pre><code>&lt;PreferenceScreen&gt;
  general settings
     &lt;PreferenceScreen android:key="adv_settings"&gt;
     more advanced settings
     &lt;/PreferenceScreen&gt;
  &lt;/PreferenceScreen&gt;
  </code></pre>
  
  <p>My problem is that the second screen doesn't show the back/up button on the action bar automatically. How do I make the up button appear on adv_settings?</p>