requestTetheredInterface() only returns a callback if an interface for
tethering is available. The previous code timed out after 500ms which
was not enough for some devices. Instead of increasing the timeout, this
change calls setEthernetEnabled() which triggers a callback as soon as
it was processed on the handler thread. Since this event is guaranteed
to be processed after requestTetheredInterface(), the subsequent
expectOnAvailable() does not have to wait at all.
Test: TH
Flag: EXEMPT test
Bug: 418151132
Change-Id: Iff119a8e425d4071bf63fe92815061c7eb36848a
3556c3d7b1