Add EventMetricProducer

+ Started to use ProtoOutputStream in EventMetricProducer.
  [TODO]: We need to auto-generate fieldIds for StatsLogReport, XXXMetricData, etc.
  [TODO]: We need to add Enum type to liblog, otherwise we cannot reconstruct a proto containing
          an enum

+ Some refactor in metric initialization code. There are still boiler plate code, because Metrics
  are similar but with subtle differences.

Test: statsd_test

Change-Id: Id7e3212566249a8139b9680f04238c455d50c1b8