Connectivity metrics: simplify ConnectivityMetricsEvent

Now that ConnectivityMetricsEvent is only used for core networking
metrics and is not @SystemApi anymore, remove unused fields and prepare
for additional new fields.

Test: updated unit tests, $ runtest frameworks-net passes
Bug: 34901696
Change-Id: I15abad19981d491f16f2a3afe401f1e833079907