title: How to especific path external-path for apps with dynamic applicationId
body: <p>I'm using FileProvider in my app, when I take a picture from the app is stored there. also my app have a different application id for debug and release builds</p>
  
  <ul>
  <li>com.rkmax.myapp</li>
  <li>com.rkmax.myapp.debug</li>
  </ul>
  
  <p>I have defined my file provider like this</p>
  
  <pre><code>&lt;provider
      android:name="android.support.v4.content.FileProvider"
      android:authorities="@string/authority_file_provider"
      android:exported="false"
      android:grantUriPermissions="true"&gt;
      &lt;meta-data
          android:name="android.support.FILE_PROVIDER_PATHS"
          android:resource="@xml/file_paths" /&gt;
  &lt;/provider&gt;
  </code></pre>
  
  <p>the value of <code>@string/authority_file_provider</code> will turn into:</p>
  
  <ul>
  <li>com.rkmax.myapp.fileprovider</li>
  <li>com.rkmax.myapp.debug.fileprovider</li>
  </ul>
  
  <p>and my <code>@xml/file_paths</code> is defined like</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;paths&gt;
      &lt;external-path name="my_images" path="Android/data/com.rkmax.myapp/files/Pictures" /&gt;
  &lt;/paths&gt;
  </code></pre>
  
  <p>if I try to changes to something like <code>Pictures</code> or <code>files/Pictures</code> my app fails</p>
  
  <pre><code>Failed to find configured root that contains/storage/emulated/0/Android/data/com.rkmax.myapp.debug/files/Pictures/20161112_073128-251269360.jpg
  </code></pre>
  
  <p>How I define a relative path in the file provider paths?</p>