IP connectivity metrics: NetworkEvents have transports

Also removes netId field from inside NetworkEvent and stop populating
the network_id field of the NetworkEvent field, deprecating it.

Bug: 34901696
Test: runtest frameworks-net,
      manually looking at $ adb shell dumpsys connmetrics

Change-Id: I2c36860f976898883714f72f4d18e06da250c6a6