NetdEventListener: add rolling log of connect and dns stats

This patch adds a rolling time order logs of basic statistics of
connect and dns, grouped by network id and including:
 - average error rate
 - average latency
 - max latency
 - total number of operations

The log is segmented in buckets of 5 minutes, and covers the last 4
hours.

Note that only blocking connect() calls latencies are recorded with the
current netd logging infrastructure.

Example of logs:
05:30:00.000: {netId=100, WIFI, dns avg=92ms max=525ms err=00.0% tot=37, connect avg=0ms max=23ms err=00.0% tot=25}
05:35:00.000: {netId=100, WIFI, dns avg=94ms max=537ms err=05.3% tot=57, connect avg=0ms max=0ms err=00.0% tot=69}
05:40:00.000: {netId=100, WIFI, dns avg=220ms max=350ms err=00.0% tot=4, connect avg=0ms max=0ms err=00.0% tot=2}
05:45:00.000: {netId=100, WIFI, dns avg=112ms max=113ms err=00.0% tot=1, connect avg=0ms max=0ms err=00.0% tot=1}
05:50:00.000: {netId=100, WIFI, dns avg=131ms max=269ms err=00.0% tot=2, connect avg=0ms max=0ms err=00.0% tot=1}

Bug: 65700460
Test: runtest frameworks-net
Change-Id: I54e76d18fbaaa92639fb675f93ea90b7615fd6bf