IP connectivity metrics: cleanup obsolete code

This patch cleans obsolete code related to IP connectivity metrics:
 - remove @SystemApi on android.net.metrics: now that metrics events
   are processed and serialized in the frameworks only, event classes
   should not appear in the system apis.
 - remove obsolete Logger classes: ConnectivityMetrics app was the
   unique user of ConnectivityMetricsLogger until nyc-mr1. From nyc-mr1
   the app started using dumpsys to get metrics for IpConnectivity and
   Telephony, which made ConnectivityMetricsLogger obsolete.
 - simplifications in MetricsTestUtil

Test: - runtest frameworks-net
      - manually verified $ adb shell dumpsys connmetrics
Bug: 30054585
Bug: 32648597
Change-Id: I85ef65f7f69eb9299e4636cc7af54067201d9daf