feat(linux): Deal with temporary dbus session

When we launch dbus if it is not running, we're probably (un-)installing
keyboards for a different user. This is an unusual environment where not
everything is available, e.g. dconf-service won't be running and can't
be started because DISPLAY variable is not set. This means we can't
write gsettings values through the API. However, we can read them, and we
can write a keyfile and update the database. This is what this change
implements for the cases where we started dbus.