you modified the manifest and added "MANAGE_EXTERNAL_STORAGE" manually.. .
which results in your app not being accepted by Google Play anymore...
you should mention that next time in your first post...
did you use the AskForPermission method together with MANAGE_EXTERNAL_STORAGE?
also you might want to try one of the file extensions to get a list of the files of that directory...
The guide / tutorial is about "How to request and grant MANAGE_EXTERNAL_STORAGE permission" and it is not about not making this request every time the app is restarted. So if this permission is granted once, the app will have unrestricted access to all files, even if the app is restarted.
So once this permission has been granted there is no need to re-submit this request.