FileProvider training fix: add note about Support Lib to setup lesson

    implementation of {@link android.support.v4.content.FileProvider} to your app, and how to

    specify the files you want to offer to other apps.

</p>

<p class="note">

  <strong>Note:</strong> The {@link android.support.v4.content.FileProvider} class is part of the

  <a href="{@docRoot}tools/support-library/features.html#v4">v4 Support Library</a>. For information

  about including this library in your application, see

  <a href="{@docRoot}tools/support-library/setup.html">Support Library Setup</a>.

</p>

<h2 id="DefineProvider">Specify the FileProvider</h2>

<p>

    Defining a {@link android.support.v4.content.FileProvider} for your app requires an entry in