Adding middleware to support EthernetSettings

Adds EthernetInterface class and its unit tests. This class encapsulates
all operations performed on an EthernetInterface. EthernetManager and
ConnectivityManager are its dependencies to execute operations.

Flag: com.android.settings.connectivity.ethernet_settings

Test: atest SettingsRoboTests:com.android.settings.network.EthernetInterfaceTest

Change-Id: I640a607c8d9705a10dc4c4c609e036a1afa6b5f7