[pkg-apparmor] Bug#865206: abstractions should allow Flatpak exports directories

[intrigeri]

Control: retitle -1 Abstractions should allow Flatpak exports directories

My initial attempt¹ was buggy and incomplete, as Simon noticed
(thanks!). Someone should come back to it, implement Simon's
suggestion and test it against a real Flatpak installation.
That someone could be me but everyone else should feel free to take
it: just let me know so we avoid duplicating work :)

[1] https://code.launchpad.net/~intrigeri/apparmor/flatpak-exports/+merge/331056