Refactor IP connectivity event logging

This patch removes static methods for logging IP connectivity events
defined in android.net.metrics and replaces them with a single log()
instance method defined on IpConnectivityLog. Event constructors are
now public also. Every classes logging such events now create an
instance of IpConnectivityLog for logging event objects directly
instantiated with new.

Removing static dependencies allow straightforward testing of logging.

This patch also removes the base IpConnectivityEvent class which is not
needed any more.

Bug: 29035129
Change-Id: I3de700f93f46deaa48a759f938f7d00e1d8bff98