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: Id0d536ff723ded5c26eafe0bb138ba75ba2856c5
Merged-In: I4769496383943e714a1d350c298e093c2ed57477

(cherry picked from commit dfc2cc58)