IpConnectivityMetrics: rate limit ApfProgramEvents

This patch uses the previously introduced TokenBucket to rate limit
ApfProgramEvents, still allowing for burst of ApfProgramEvents when a
new interface is set up (due to ipv4 provisioning, multicast lock, ipv6 RAs
triggering new APF program events in short amounts of time).

Test: new test in IpConnectivityMetricsTest
Bug: 1550402
Change-Id: I1a544a8d62e6b02f0ed0088ab76c4bad338d2f44