Connectivity metrics: add transports to validation probes

This patch adds transports info to ValidationProbeEvent and migrates
netId logging for this event to the topt-level netId field in
ConnectivityMetricsEvent.

Test: modified unit tests. $ runtest frameworks-net passes
Bug: 3490169
Change-Id: Ibf51049ba8901ae5ca4ea86e2f500944a4738b5c