1. If DHCP fails, set the score to 0. Coupled with changes in
NetworkAgent, that will cause us to retry DHCP the next time
the link is plugged in.
2. Send LinkProperties before sending NetworkInfo, because of
a race in NetworkAgent.
3. Make Ethernet work properly after a runtime restart.
4. Improve locking.
5. Clarify how things work in comments.
Bug: 15295359
Change-Id: I06cd683a1633838bca1ec177e61f1829889d3934