EthernetTetheringTest: join handler thread after calling quitSafely

Somewhat surprisingly, HandlerThread#quitSafely is *not* a synchronous
method, so the tearDown() method still needs to wait for the thread to
join.

Test: TH
Change-Id: I4297ba057b3d4e67c7b462b6995da637f2479d42
1 file changed