Notify success before configuring the IP address.

This ensures that IpManager has a complete copy of DHCP state
before configuring the IPv4 address. That way, when the netlink
notification of the IPv4 address arrives, we are guaranteed to
have all DHCP information (including DNS servers and routes)
available and we will never send the IPv4 address before the DNS
servers.

Bug: 27605330
Change-Id: I732424dfe2408be9e40028563e99d610b541e12d