Adding EthernetInterfaceTracker to support EthernetSettings

This class is part of the middleware and will be used to be notified
when an EthernetInterface is added or removed.

Flag: com.android.settings.connectivity.ethernet_settings

Test: atest
SettingsRoboTests: com.android.settings.network.EthernetInterfaceTrackerTest
Change-Id: If3a1f4d7a96b301cd47ffbea55864bcc780cd1e0