Move EthernetNetworkFactory to using the NetworkProvider API

This CL makes EthernetNetworkFactory inherit from NetworkProvider rather
than NetworkFactory. The name of the class is purposefully unchanged to
make the code review easier (it will be changed to
EthernetNetworkProvider in a follow up).

As part of the conversion, NetworkInterfaceState now registers a
NetworkOffer when the link comes up and unregisters it when the link
goes down. It updates the existing offer when capabilities change (by
calling registerNetworkOffer with an already registered
NetworkOfferCallback).

This change should fix existing refCount issues. When a NetworkOffer is
first registered, it receives callbacks for all existing requests. This
is the main problem with the NetworkFactory implementation where only
one NetworkOffer is registered when the factory is first created; so
when interfaces come up, they do not receive callbacks for existing
requests.

Test: atest EthernetNetworkTest
Bug: 197548738
Change-Id: I5e8e4673d2ed04bc1a0c8d232a8772edfff65b5d