Rework flatpak for MM

This is not tested on a real device, and it's very possible it doesn't completely work. The daemon likely doesn't work, I'm not sure how something like that should be done with flatpak. We could probably use the background portal: https://flatpak.github.io/xdg-desktop-portal/#gdbus-org.freedesktop.portal.Background

It would be cool if we could publish that to flathub in the end, and this goes a large part of that way.