Fix EthernetTetheringTest failure if tethering is not supported

Do not run ethernet tethering tests if tethering is not supported
because Tethering APIs would fail. Also skip the test if
EthernetManager is not avaliable. EthernetManager would be null if
there are no FEATURE_ETHERNET and FEATURE_USB_HOST.

Bug: 159869957
Test: atest CtsTetheringTest
Change-Id: I2b9be6799c6edeefc8cd74897a8704dbe80dd061