main/postmarketos-base-ui: move flatpak repo config to /usr

Since 1.15.11 this is supported (see https://github.com/flatpak/flatpak/pull/5688/files). According to the docs:

System-wide remotes can be statically preconfigured by dropping flatpakrepo files into /usr/share/flatpak/remotes.d/ and /etc/flatpak/remotes.d/. If a file with the same name exists in both, the file under /etc will take precedence.

This allows us to move the file without fear that if users disabled or modified the repo, apk will keep the file in /etc and we won't override it.

This is part of a wider effort to move things out of /etc and into /usr

Fixes #2596 (closed)