abstractions/freedesktop.org: treat Flatpak exports the same way as bits shipped by the distro
This supersedes https://code.launchpad.net/~intrigeri/apparmor/flatpak-exports/+merge/331056 and addresses the issues raised by Christian and Simon there. I've tested this both with a system-wide Flatpak and a per-user one. Without this change I can reproduce the denial reported on the Debian bug; with this branch merged I can't reproduce it anymore.
Candidate for 2.13 and master.