refactor download manager

[theScrabi]

• [5] I carefully read the contribution guidelines and agree to them.
• [no] I checked if the issue/feature exists in the latest version.

Gigaget wase once a downloader distributed on fdroid. However it is discontinued, tho we copypasted their code into NewPipe, and made gigaget our downloader. Now we have to get rid of it, since its causing a lot of trouble. Related issues: https://github.com/TeamNewPipe/NewPipe/issues/961 https://github.com/TeamNewPipe/NewPipe/issues/779 https://github.com/TeamNewPipe/NewPipe/issues/355 https://github.com/TeamNewPipe/NewPipe/issues/345 https://github.com/TeamNewPipe/NewPipe/issues/315

Maybe we could replace it with a Download Manager available on Android Arsenal. Feel free to browse a little and find a downloader you think fits to NewPipe.

A new download manager must have these abilities:

• using SAF (for downloading to sdcard)
• Be singe threaded (one thread per download)

Nice to have:

• Prevent/restrict download on mobile connection
• ~~download files with external download manager~~ #9142
• easy accessing videos form download manager
• make it a lib and put it back on android arsenal
• show download status in titlebar/notification