Use new DHCP server in TetheringTest

Run most tests with TETHER_ENABLE_LEGACY_DHCP_SERVER set to 0 (will be
the default value). Add one test to verify that the new server is not
started when TETHER_ENABLE_LEGACY_DHCP_SERVER is 1.

Bug: b/109584964
Test: atest FrameworksNetTests
Change-Id: I288ba1f434918e62ff29f7ace00856108c9730f7