Updating Eth CTS to account for existing ifaces

Updating EthernetManagerTest to account for existing ethernet
interfaces. Specifically, testCallbacks() assumed the test interface
created would be the only ethernet interface on the device. This would
cause the test to fail if other ethernet interfaces existed.

Bug: 230305098
Test: atest CtsNetTestCases:android.net.cts.EthernetManagerTest
Change-Id: I675c4ef5c2bd96d01a057bdfec1794d6bfcf82bb
1 file changed