Extract logging of default network events

This patch extracts the logging of DefaultNetworkEvent from inside
ConnectivityService and move it to a new DefaultNetworkMetrics class.

The DefaultNetworkMetrics is a singleton owned by the
IpConnectivityMetrics singleton implementing the metrics service for
core networking. ConnectivityService has access to this singleton via
LocalServices.

This class layout will allow to remove the Parcelable interface of
DefaultNetworkEvent and will instead let the IpConnectivityMetrics
service grab metrics from the DefaultNetworkMetrics directly.

Bug: 34901696
Test: runtest frameworks-net
Change-Id: I55694d89124272732aba114198776462372de18b