Show notification progress when downloading a file through SAF document provider

### Is your feature request related to a problem? Please describe.

Consider the following: you're trying to access a non-small file (~100MB or greater) through SAF. If the file has not been downloaded, the main app might hang, waiting for the file to be downloaded.

I encountered such problems on Emacs for Android. Opening a 70MB file results in the app hanging without knowing when it will ever end (it eventually did)

### Describe the solution you'd like
Show a push notification when a file is getting downloaded. This might even help people with limited data usage, so they can be more aware of their network usage.