DHCP: Minor cleanups to the packet code.

1. Delete the DhcpStateMachine, since we don't plan to use it.
2. Make all InetAddresses Inet4Addresses, since that's what they
   are.  In order to do this, define INADDR_ANY and
   INADDR_BROADCAST, constants, since Inet4Address.{ANY,ALL} are
   not Inet4Addresses but InetAddresses.

Bug: 19704592
Change-Id: I5a0499be889076992a60aaad0bd8be5ea66bd560