IP connectivity metrics: ConnectStats additions

This patch:
  - adds a counter to ConnectStats for counting the number of successful
    connect() calls done in blocking mode. This allows to compute error
    rates for connect() in blocking mode.
  - add printing of ConnectStats when dumping NetdEventListenerService
    and/or IpConnectivityMetrics service.

Test: $ runtest frameworks-net, + manual tests verifying logged events.
Bug: 34901696
Change-Id: I889e8ccd12681b0511be487e9d2ee44290a2d7d7