ConnectivityMetricsLogger subclass for IpConnectivity

IpConnectivityEvent was using ConnectivityMetricsLogger directly for
logging events. However ConnectivityMetricsLogger keeps track in a
thread-unsafe way of skipped events rejected by MetricsLoggerService.

This patch introduces a subclass of ConnectivityMetricsLogger that
does not track skipped events, for using in IpConnectivityEvent.

It also qualifies the mServiceBlockedTimestampMillis variable as
volatile so that throttling is effective accross concurrent callers
of logEvent.

Bug: 28204408
Bug: 29023888
Change-Id: I33707ba1d07487b42f3ce9a1ad9a66d785e99fa7