I am able to build many android apps and a fdroid compatible repo with nix repo/app at main · mio-19/repo · GitHub
I sign the repo with my key and host it on my lan. My android devices happily download apps with a fdroid client.
But there is one problem when building android gradle projects. With nixpkgs’s gradle infrastructure the generated lockfiles often contain urls that change every day Request: gradle fetchDeps overriding url · Issue #501643 · NixOS/nixpkgs · GitHub