The tracking interface of EthernetTracker(mTetheringInterface) is
stopped to track only when it is removed. There is race condition in
EthernetTetheringTest that the new test start ethernet tethering with
previous test interface name because EthernetTracker still tracking
previous test interface.
Destroy test interface then listen untether callback without calling
stopTethering. If EthernetTracker stop tracking test interface, it would
call onUnavailable callback to stop tethering. So the test can check
whether tethering is stopped to ensure the test interface is not tracked
anymore before test finish.
Bug: 191826409
Bug: 243222655
Test: atest TetheringIntegrationTests
Change-Id: I618983d981cd3a707dcb7723cdbb20ba5bf8bd2e
f8371fef0f