There is a race between releaseTetheredInterface() and a new interface
being created. Calling setEthernetEnabled(true) forces the test to wait
on a callback that is generated on the ethernet handler thread (and
therefore guarantees releaseTetheredInterface() has been processed).
Bug: 296012516
Test: atest EthernetManagerTest
Change-Id: I7a1a763ba5c1c50274a112b8c1b8c04f35696f12