instance: Be more defensive in get_child_pid
instance: Be more defensive in get_child_pid

There is a race; flatpak_installation_launch may return
before bubblewrap has written the bwrapinfo.json file.
Make flatpak_instance_get_child_pid() deal with this and
document that the function may return 0 for a little
while.

flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak