title: How to set Preferences 's default value of checkbox
body: <p>My app has preference which has checkbox and it's default value is "unchecked"</p>
  
  <p>How can I set it to "check"?</p>
  
  <pre><code>&lt;CheckBoxPreference
          android:key="checkbox6"
          android:title="setting"
          android:defaultValue="true"
      /&gt;
  </code></pre>
  
  <p>and it still doesn't "check" at the first time what should I do? </p>