Fix test failure on Android TV due to built in ethernet interface

Android TV is failing on
EthernetManagerTest.testSetTetheringInterfaceMode_disableEnableEthernet
due to its built in ethernet interface is identified as tethering
interface.
This patch mark the test as only run when no tethering interface
available.

Bug: 389829357
Test: atest EthernetManagerTest
Change-Id: Iab159e224d6baef8ae55ecc2d91a659b90a31538
1 file changed