Add metric computation skeleton to statsd.

This cl is to let statsd understand statsd_config, and compute metrics
 defined in the config.

+ StatsLogProcessor is given a StatsdConfig (hard coded right now).
  We construct a MetricProducer for each of the metric, and the metrics
  share Condition and LogEntryMatchers

+ Added the CountMetricProducer type for CountMetric.

We can now count times of SCREEN_ON events given a config.

TODO: 1) conditions are not implemented.
      2) slicings are not implemented in CountMetric
      3) move the interaction to dropbox to a separate thread
      4) decide how the in memory metrics would be used by anomaly detection

Test: manual test.

      $ adb shell /system/bin/statsd

      $ cat config_file.dat | adb shell cmd stats config

Change-Id: I38f4059c0dc5a827c338131d4a6fa7d4cbe865db