Skip testPhysicalEthernet if the test run adb over network

If the test run adb over network and ethernet is available, it is likely
that adb may run over ethernet. Then the test would fail because adb
would break when ethernet is switching from client mode to server mode.

Bug: 160389275
Test: atest CtsTetheringTest

Change-Id: I57d365d33316881c50c3bf7fd1c98926d10842d3