statsd: parse the new format of stats log

+ Changed how we construct LogEvent, now it's based on the context from log_msg
  without making a copy of the list.

+ All stats logs now have the same event tag, the atom id is the first elem in the log.

Test: statsd_test
Change-Id: I4419380f2ee1c2b2155d427b9f2adb78883b337f