IpConn metrics: distinguish NUD_FAILED answers

This patch adds in IpReachabilityMonitor a timestamp variable set
everytime that probeAll() send NUD probe requests to RTNETLINK.
This allows to distinguish between:
  1) NUD_FAILED events resulting from such a forced NUD probe
  2) "organic" NUD_FAILED notifications from the kernel

This distinction is added to IpReachabilityEvent as a one-bit flag.

This patch also changes the formatting of ApfProgramEvent flags to use
'|' as a joining character, similarly to other flags formatting.

Bug: 21859053
Change-Id: I24c64a3f17fa283eace5bd0a05c21a90a2305359