This is a no-op, test-only change.
Move EthernetStateListener from EthernetManagerTest.kt to a new
standalone file in testutils.
This refactoring makes the listener reusable across other tests and
decouples it from its dependency on EthernetTestInterface.
Flag: EXEMPT refactor
Test: atest CtsNetTestCases:android.net.cts.EthernetManagerTest
Bug: 410731080
Change-Id: Iacd9236f758011da0714884040aebe3e42ef1646