DHCP: Add a superclass for DhcpStateMachine.

This can be used to switch between different DHCP client
implementations. The caller can declare objects of type
BaseDhcpStateMachine, and call its methods, without needing to
care what implementation is in use.

Bug: 19704592
Change-Id: Icefad9b0d0f83b349681388b1fa16b5e2e37c042