title: What is "android:key" in <PreferenceScreen> used for?
body: <p>In this example XML Layout of a <a href="http://developer.android.com/reference/android/preference/PreferenceScreen.html" rel="noreferrer">PreferenceScreen</a>:</p>
  
  <pre><code>&lt;PreferenceScreen
          xmlns:android="http://schemas.android.com/apk/res/android"
          android:key="first_preferencescreen"&gt;
      &lt;CheckBoxPreference
              android:key="wifi enabled"
              android:title="WiFi" /&gt;
      &lt;PreferenceScreen
              android:key="second_preferencescreen"
              android:title="WiFi settings"&gt;
          &lt;CheckBoxPreference
                  android:key="prefer wifi"
                  android:title="Prefer WiFi" /&gt;
          ... other preferences here ...
      &lt;/PreferenceScreen&gt;
  &lt;/PreferenceScreen&gt; 
  </code></pre>
  
  <p>I noticed an <code>android:key</code> for both PreferenceScreens (nested in each other).</p>
  
  <p>I couldn't find, however, any use for them in application code: The only keys used to access a value by program code are the ones of the actual elements: CheckBoxPreference, EditTextPreference, etc.</p>
  
  <p>Can they be utilized for specifying in code "access the CheckBoxPreference that's in this PreferenceScreen and not the other"? i.e. hierarchical access?</p>
  
  <p>If not, what is the purpose of the key for a PreferenceScreen? Must it be defined? Must it be unique?</p>