Make Ethernet interfaces more testable.

This CL adds a setIncludeTestInterfaces method to EthernetManager
that, when called, causes the Ethernet service to recognize and
manage test interfaces created by TestNetworkManager.

Bug: 150644681
Test: Tested by EthernetTetheringTest in same topic
Change-Id: I86eef7a93267f800dbfc8eafd307effa76a344ca