Add util to add an ARP table entry

This is to be used by the new DhcpServer to add ARP entries with new
addresses before sending unicast responses.

Test: manual: cat /proc/net/arp with implementation based on this
Bug: b/109584964
Change-Id: I3559893583aa3c49b188ad689a41ee2f3e9d9bf3
2 files changed