Log RA listening statistics

This patch adds a new ApfStats event class that counts RA packet
reception statistics on the RA listener thread of ApfFilter and reports
the maximum program size advertised by hardware.

Statistics are gathered for the lifetime of a network with APF
capabilities and uploaded at network teardown when the listener thread
exits.

Example event:
ConnectivityMetricsEvent(15:44:23.741, 0, 0): ApfStats(284945ms 2048B RA: 2 received, 0 matching, 0 ignored, 0 expired, 0 parse errors, 2 program updates)

Bug: 28204408
Change-Id: Id2eaafdca97f61152a4b66d06061c971bc0aba4c