New IpConnectivityMetrics service

This patch defines a new metrics service for IpConnectivity events
defined in android.net.metrics, separate from currently existing
MetricsLoggerService.

Similarly to MetricsLoggerService, the new service has an event buffer.
It also implements a dumpsys interface that can be used to flush events
and output a serialized proto.

Bug: 31254800
Change-Id: I0c3faeb4008b283f85d9ba9460371fa68956ea3b