Actually fall back from yiaddr to ciaddr.

The initial implementation of toDhcpResults attempted to get the
leased IP address from ciaddr if yiaddr was 0.0.0.0, but it never
actually did so because a) it used == instead of equals(), and b)
the parsing code never populated mClientIp for a DhcpOfferPacket
or DhcpAckPacket.

Fix this and add a test for it.. Technically DHCP does not use
ciaddr (only bootp uses it), but in 5.0 we would use ciaddr if
yiaddr was 0.0.0.0 and a bit more compatibility shouldn't hurt.

Bug: 19704592
Change-Id: I1f58555f0c10b9c576995a6edb759a83d8938ea0