Tcp socket metrics: implement INetdEventListener callback

This patch implements the new INetdEventListener.onTcpSocketStatsEvent
callback added in INetdEventListener.

For the time being, tcp socket stats are tracked inside TcpMetrics only
for dumpsys printing and bug report integration as a first step.

Bug: 64147860
Test: manually tested, watching output of
      $ adb shell dumpsys connmetrics
Change-Id: I10ab24c6da4bb654d9198a4d8d00ccdc972cc0d5