title: How to add the category on preference header?
body: <p>I have a simple <code>preference_headers.xml</code> file. I want to add some categories on my preference headers. I have tried to implement <code>&lt;PreferenceCategory /&gt;</code> in my <code>preference_headers.xml</code>, but it doesn't work. I'm using API level 11. As shown in the figure below, Android Developers from <a href="http://developer.android.com/guide/topics/ui/settings.html#PreferenceHeaders" rel="noreferrer">official website</a> can add the categories. How can I add it too?</p>
  
  <p><img src="https://i.stack.imgur.com/AAWcR.png" alt="categories on preference header"></p>
  
  <p>And here is a few of my <code>preference_headers.xml</code> code. It won't work:</p>
  
  <pre><code>&lt;preference-headers xmlns:android="http://schemas.android.com/apk/res/android"&gt;
  
      &lt;PreferenceCategory android:title="ACCOUNT" &gt; 
  
          &lt;header android:fragment="com.example.preferenceheader.SettingsFragmentAccount"
              android:title="@string/pref_header_category_account"
              android:summary="@string/pref_header_summ_account"
              android:icon="@drawable/ic_action_settings" /&gt;
      &lt;/PreferenceCategory&gt;
  
      &lt;PreferenceCategory android:title="APPLICATION" &gt;
  
          &lt;header android:fragment="com.example.preferenceheader.SettingsFragmentApplication"
              android:title="@string/pref_header_category_application"
              android:summary="@string/pref_header_summ_application"
              android:icon="@drawable/ic_action_group" /&gt;
      &lt;/PreferenceCategory&gt;
  
  &lt;/preference-headers&gt;
  </code></pre>
  
  <p>Thanks for your help.</p>