exported-object: set object path after skeleton creation

exported-object: set object path after skeleton creation

When exporting an object, we first set the object path and then create
GDBus interface skeletons. While doing this, a signal can be generated
[1] and then nm_exported_object_signal_hook() can trigger the failed
assertion "interface != NULL" because the object is already exported
(priv->path != NULL) but the interface has not been registered yet.

To fix this, set the object path only after skeletons have been
created.

[1] This happens here every time I disable networking and restart NM:
    #0  _g_log_abort (libglib-2.0.so.0)
    #1  g_log (libglib-2.0.so.0)
    #2  nm_exported_object_signal_hook (NetworkManager)
    #3  signal_emit_unlocked_R (libgobject-2.0.so.0)
    #4  g_signal_emit_valist (libgobject-2.0.so.0)
    #5  g_signal_emit (libgobject-2.0.so.0)
    #6  set_state (NetworkManager)
    #7  nm_manager_update_state (NetworkManager)
    #8  get_property (NetworkManager)
    #9  object_get_property (libgobject-2.0.so.0)
    #10 on_source_notify (libgobject-2.0.so.0)
    #11 g_object_bind_property_full (libgobject-2.0.so.0)
    #12 g_object_bind_property (libgobject-2.0.so.0)
    #13 nm_exported_object_skeleton_create (NetworkManager)
    #14 nm_exported_object_create_skeletons (NetworkManager)
    #15 nm_exported_object_export (NetworkManager)
    #16 nm_manager_setup (NetworkManager)
    #17 main (NetworkManager)
    #18 __libc_start_main (libc.so.6)
    #19 _start (NetworkManager)

https://mail.gnome.org/archives/networkmanager-list/2016-February/msg00041.html