dhcp client: use arp to find ip address collisions

This allows the dhcp_client to check if the offered ip
address from dhcp server is being used by others.
While there, this also rename ResetStateVariables() to
ResetState().

Bug: 25642025
TEST=compile, and test using python scripts

Change-Id: Ib5d982943d0be47178039c7a6f36b44eac9133f5
2 files changed