@Cambion At the moment, not natively, but some of our community has suggested using Obtainium to do this: https://www.reddit.com/r/ProtonMail/comments/1d1o3qw/comment/l5wa83c/

Bear in mind that this method has not been officially tested nor is it endorsed by us, so if you do decide to try it, use it at your own risk.

F-droid availability is planned, but we don't have a specific timeline to confirm at the moment.