Switch DHCP server based on global setting

The newer implementation is disabled by default with this CL. Ultimately
the intention is to enable it by default.

Bug: b/109584964
Test: set tether_enable_legacy_dhcp_server to 0, ran DhcpServerTest.py,
      observed new behavior. Added tests in CL also pass.

Change-Id: I0f830b9804b8956c127057e66ab75a21ca29dc57