Fix missing IpConnectivity metrics

The IpConnectivityLog class looks up MetricsLoggerService once only
at creation. If a IpConnectivityLog user instantiates this class too
early during the boot process, the MetricsLoggerService is not found
and no event can be recorded.

This patch makes IpConnectivityLog attempt to look up
MetricsLoggerService as long as it hasn't found it yet.

This allows IpManager and ConnectivityService to upload
android.net.metrics events.

Bug: 30490301
Change-Id: I97102b95a775ea9e90351b9887ae4661fddc2af9