IpConn metrics: correctly read RA lifetimes

This patch
  - adds a Builder class for RaEvent.
  - uses this Builder class for correctly recording the minimum
    lifetime seen for every ICMP6 options tracked, instead of
    recording the last lifetime seen.
  - adds unit test coverage for RaEvent logging.

Change-Id: I6443932f5cf7a613a5c695c65a60eab01e60602a