Send ciaddr in DHCP server ACK if set by client

RFC2131#4.3.1 specifies that ciaddr should be set to the "'ciaddr' from
DHCPREQUEST or 0". Previous behavior sets ciaddr if provided by the
client in DHCPREQUEST, so keeping this behavior.

Bug: b/109584964
Test: DhcpServerTest.py debian/macos/win10 renewing test pass
Change-Id: I283e88b1f66f39d24589c5af4518cf9a2e77b95d