Add DhcpServer

This first version can serve discover/request/release, although there
are some small behavior changes with current implementation which will
be addressed later.

Also removes final modifiers on start() and stop() in FdEventsReader, to
allow mocking the methods in tests with the current mockito lib.

Test: Added tests pass, manual: flashed a device using the server
Change-Id: I025366ff7d51c4ba31152af50f3dd2b5e280a54d