TestOnly: add setEthernetEnable support to EthernetManagerTest

Also fix the bug in teardown that setIncludeTestInterfaces(false)
should be called after the interfaces are destroyed, otherwise
the call to ifaceListener.eventuallyExpect(ABSENT) may receive
callbacks out of order (and it doesn't actually wait on the
interface being removed properly).

Test: atest EthernetManagerTest
Bug: 247964480

Merged-In: I6650c028c121d932f10219d04d40d1ad60d9d4d8
Change-Id: I6650c028c121d932f10219d04d40d1ad60d9d4d8
(cherry picked from commit bd5745ef)