Tcp socket metrics: define INetdEventListener callback

This patch adds a onTcpSocketStatsEvent() callback method to the
INetdEventListener binder interface that the system server implements.

This function is called by netd for every sock_diag poll performed and
is used to reports to the system server tcp socket stats per network.

Bug: 64147860
Test: manual tests
Change-Id: I0f3c96b3b6e0e2647e662cb61ef16ee7814eac59