library for direct self-updating from F-Droid repos

Lots of apps have the ability to directly download and install updates for themselves, in case there is no available update channel (e.g. F-Droid, Google Play, etc). There are also libraries to do this, including https://github.com/javiersantos/AppUpdater. We should make a library to do this based on F-Droid repos. This would be something that would work in conjunction with #714 (closed) to get updates flowing as easily as possible while nudging users towards installing F-Droid when they are most receptive to the idea, since fdroidclient will provide the best update experience (Tor, only on Wifi, etc).

We now have an awesome use case for it: official Firefox updater: https://bugzilla.mozilla.org/show_bug.cgi?id=1192279