Refactor DHCP server with StateMachine.

To support DHCPDECLINE message and request a new prefix from
IpServer, a WaitState is required to wait until IpServer allocates
a different prefix and completes configuring this prefix/route.
Then server could resume from pausing DHCP packets listening.
From this point, StateMachine is easier to add a WaitState for
implementation. Refactor DHCP server by replacing ThreadHandler
with StateMachine first.

Bug: 130741856
Test: atest NetworkStackTests NetworkStackNextTests
Test: manual test: connect wifi, turn on hotspot, downstream
      device attaches to hotspot successfully, then turn off
      hotspot, repeat multiple times.

Merged-In: I6c09d9c371e9c4e71d8ba26adaed640e3b97437b
Change-Id: I6c09d9c371e9c4e71d8ba26adaed640e3b97437b