ethernet: cleanup ethernet state listener interaction

Add a addEthernetStateListener() function that works just like
addInterfaceStateListener(). Make sure ethernet state listeners are
added to addedListeners, and addedListeners are removed from both
ethernet and interface state listeners on tearDown. This is safe even if
a given listener was not registered as one or the other.

Test: atest EthernetManagerTest
Change-Id: I74d9d0747b94b7d34a2fe7630e840acd34f1a500