Canceling an upload must not remove the files from the upload list

### Actual behaviour
- when the app is uploading or if it shows a list of failed uploads the user can hit the "X" at the top right. This will remove the files currently uploading or those that failed from the list and thus from ever being uploaded.

Without any dialog.

### Expected behaviour
- hitting the "X" must only cancel the transfer not remove the files from the list. Especially not without warning the user.

### Steps to reproduce
1. Wait until the app is uploading a number of files
2. Hit the "X"
3. Check the upload list for the files not uploaded

### Environment data
Android version: 8.1

Device model: Nexus 5X

Stock or customized system: lineageos

Nextcloud app version: 3.6.0

Nextcloud server version: 15.0.6