Comment 1 for bug 1620442

We need this for dconf.

See https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html for info about the general requirements for the directory.

In particular, for dconf's case:

  - a new directory should be created when an app is run, and it should be shared
    between instances of that app (to the extent that this makes sense in the snappy world)

  - the app should not be able to see the external XDG_RUNTIME_DIR

  - the directory that is created (or some part of it) needs to be visible to the outside world
    flatpak does this by bindmounting XDG_RUNTIME_DIR/app/appid/ such that this directory is the
    same on the inside and outside.