freedesktop-dbus: Try to own the name until after exporting skeleton

Currently, the g_bus_own_name_on_connection function is called for the org.freedesktop.FileManager1 name before exporting the /org/freedesktop/FileManager1 skeleton. This seemingly works fine in most cases, but occasionally the name is acquired too early and D-Bus clients can get No such interface error. This is regression caused by the commit 2293e813. Let's try to own the nam until after exporting the skeleton to avoid this error.