Move $XDG_RUNTIME_DIR/flatpak-info to /.flatpak-info and make it read-only
Move $XDG_RUNTIME_DIR/flatpak-info to /.flatpak-info and make it read-only

The old location keeps working because we create a symlink.

The interesting part of this is that the sandbox now can't modify,
remove or replace the file, and it is in a fixed position. This means
that given a sandboxed pid, we can find this file as
"/proc/$pid/root/.flatpak-info".

This is very nice for two reasons:
 * We can use this instead of the cgroup to find out if a pid is
   sandboxed or not. This means we can drop the dependency on
   systemd --user for this.
 * We can get more information about the peer process that the
   app id. For instance, we can see what permissions the sandboxed
   app has.

matthiasclasen/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak