Set device owner to router when adding a port to a router
Also, return empty string when device owner is not set

Change-Id: I9a4eb4e402622c4ec8a6179921cf4e7e7ee2bd39
Closes-Bug: 1361519
Closes-Bug: 1362286
(cherry picked from commit b9b22aaa74cfaf4c95fb2f240dad02a932ed6bdd)