Enable read and write access to root of external SD cards via Document Tree permission

### Expected behaviour
- Tell us what should happen
Read and Write of Root of External SD
Have a popup requesting and granting Document Tree access to External SD cards for the /mnt or

### Actual behaviour
- Tell us what happens
SD Card is mounted but nextcloud app has read access to External SD so therefore can only read the SD card. Nextcloud is detecting either a Manufacturer partition from the NaN or internally mounted SD card (on my phone /storage/emulated/0) as the external and has write access. Even through Nextcloud has been granted write access to the External SD card. On my phone SD cards are mounted /storage/0000-0000(In my case) If you mount a slower speed/differed compatibility sd card it will randomly selected show letters and number in the place of the zeros (ie /storage/1A3B-2C9D). The app should have a grant access to the document tree selector in /storage/xxxx-xxxx location built into it. After uploading it would be removed the actual external SD Card from the phone and stored on the cloud only.

### Can you reproduce this problem on https://try.nextcloud.com?
- Please create a test demo account and see if this still happens there.
- If yes, please open up a bug report
- If not, please verify server setup and ask for help on forum

### Environment data
Android version: Oreo 8.0.0 LS993ZVF
Device model: LG G6
Stock or customized system: Stock
Nextcloud app version: 3.11.1
Nextcloud server version: 18