How did you create folderUri?
It's the persistable uri from the Folder picker.
The problem is I also have a File picker using android.intent.action.GET_CONTENT and because they're using the same System UI/Activity the starting folder shown when it's launched is always the one last used. What I wanted was when the File Picker is launched the starting folder is the one where File Picker left off or a pre-set location. The same goes for the Folder Picker.
Anyway, I'll just make my own file/folder picker based on your sample project for the ExternalStorage class. I find using the system File Picker problematic if the user picks a file using third party file providers like TotalCMD, ZArchiver, etc., the ones on the side drawer of the file picker.