title: Permission for reading/writing to the root folder in Android
body: <p>Are there any permissions I can/must declare in my app,<br>
  if I want the app to be able to scan the whole "/" folder<br>
  (say to search for all <code>.txt</code> files e.g. recursively)?</p>
  
  <p>I am getting just <code>null</code> so far (when calling <code>File.listFiles</code>)<br>
  no matter what permissions I try. I guess I am missing something.   </p>