Add support for Ethernet tethering

Ethernet tethering can be started via
startTethering(TETHERING_ETHERNET).

Test: flashed, enabled ethernet tethering, verified internet access on
      downstream.
Bug: 130840861

Merged-In: I34842acd94b972e440c3622f7617df10c18acf65
Change-Id: I34842acd94b972e440c3622f7617df10c18acf65
(cherry-pick with conflicts in test-current.txt)