logd: Add minimum time bucket statistics

* Only enabled for dev builds because halves performance.
- Used to establish if /proc/sys/net/unix/max_dgram_qlen
  is appropriate for the platform
  (see logd/LogStatistics.cpp comments)
- enabled if logd.dgram_qlen.statistics is not zero/empty

Change-Id: Ib8ecfeb75b6f0f954c099a6b1032b8afb5efcbd4