Connectivity metrics: serialize networkId, transports, ifname

This patch adds translation from ConnectivityMetricsEvent to
IpConnectivityEvent of recently added fields:
 - top-level network id
 - top-level ifname
 - transports

Also adds inference of link layer from transports or ifname.

At the moment these new fields are not populated in
ConnectivityMetricsEvent. Follow-up patches will fill this gap for
the events of the android.net.metrics package.

Test: new unit tests, $ runtest frameworks-net passes
Bug: 34901696
Change-Id: I563a6a3183470bdfaabb7c781a1beaf6b1058bf0