title: Android 11 ACTION_OPEN_DOCUMENT_TREE: set initial URI to the Documents folder
body: <p>using the Scoped Storage model in Android 11 I want to give the user the ability to choose a folder, starting in the documents folder:</p>
  <pre><code>val intent = Intent(Intent.ACTION_OPEN_DOCUMENT_TREE)
  intent.putExtra(DocumentsContract.EXTRA_INITIAL_URI,    ???     )
  startActivityForResult(intent, OPEN_DIRECTORY_REQUEST_CODE,null)
  </code></pre>
  <p>The problem is, how can I generate a proper URI of the phone's documents folder? (It sits just in root / ) In the official documentation, no examples are given. I really hope there are some neat constants for all the standard locations?</p>