main/postmarketos-base-ui: move flatpak repo config to /usr (MR 6119)

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
[ci:skip-build]: already built successfully in CI