[External Pull Request]: Improve F-Droid metadata structure

I have decided to submit these changes here instead, as GitLab doesn't let you sign up without a phone number. Unfortunate. If these changes are deemed fit, they can be merged into the upstream's fdroiddata/metadata.

This commit over here makes the fdroid buildserver release all 4 arch-specific variants of the app. Some additional info are in the commit description. Corrections/corroborations from everyone are very appreciated.

I am suggesting these changes as I've noticed the F-Droid repo contains only the universal variant, whereas GitHub releases contain all four. It would be completely beneficial for the users, since now that an F-Droid client with native leanback support exists, the correct arch variants would be auto selected and installed. Plus it'll slightly reduce the app size ;)

Would be glad if yuliskov personally submits these changes as being the project maintainer.