DO NOT MERGE 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

(cherry picked from commit e1c173d2)

Change-Id: Ibe41e9a76db36ea502697a5f19fc0d91c40a3087