ethernet: test adding listener after removing server mode iface

Tests that registering a EthernetStateListener functions as expected
after requesting a tethering interface and subsequently removing that
interface.

Test: atest testCallbacks_afterRemovingServerModeInterface
Bug: 290971780
Change-Id: I7b6e1a3b3ac23f4fbdd2d20f63f83d59c4d4999d