Connectivity metrics: add transports to connect stats

This patch groups connect() events per netId. It adds netid and
transport information to serialized ConnectStatistics events.

Test: updated NetdEventListenerServiceTest
      updated IpConnectivityMetricsTest
      $ runtest frameworks-net passes
Bug: 34901696

Change-Id: I4769496383943e714a1d350c298e093c2ed57477