title: Can't use Android's PhotoViewActivity
body: <p>In Android SDK there is a class called <code>PhotoViewActivity</code>. I can navigate to it and view its source code in Android Studio.</p>
  
  <p>The class is public.</p>
  
  <p>Yet when I try to use it it, the IDE stubbornly claims it doesn't know what I'm talking about.</p>
  
  <pre><code>public class MyPhotoViewActivity extends com.android.ex.photo.PhotoViewActivity {
     // doesn't compile, can't find the base class
  }
  </code></pre>
  
  <p>How do I make it work?</p>