dhcp client: add socket to DHCPV4 class

The socket is used for sending and receiving DHCP messages.
We need to use raw socket before the IP is set up, so a
socket filter is needed for packet filtering.

Bug: 25642025
TEST=compile
Change-Id: Ia6da0f8954f99685a68ad1d58c887c76aed99399
2 files changed