Add CTS tests for EthernetNetworkProvider changes

Adds CTS test to validate behavior of EthernetNetworkProvider API.
These tests are currently still pretty slow due to the fact that newly
registered tracked interfaces are brought up, back down, and back up
before they are available (this is a bug and will hopefully get fixed in
the near future).

Test: atest EthernetManagerTest
Bug: 197548738
Change-Id: I8e806b3b884f2e0b6c1a1d2fffdb9a99c5dd60e8