Add support for dimension, and link with condition and added DurationMetric

Now we support following metrics:
  <Duration> of [app holding a wake lock], while [*this app*] is [in background] [AND] [screen is off]
  [Slice] the output by [app name, wake lock name], with bucket size [30sec]

+ Also added onDumpReport() api in MetricsManager, it can be called from client to fetch the data from
  statsd

+ Also added command line tool to dump the StatsLogReport from all metrics for debugging.

+ Synced proto from google3. with a pending cl (cr/172359050)

TODO: We need to add tons of tests to test the Metrics. I will work on it after this CL so people
can be unblocked.

I locally test the duration metric with wake lock with an app that generates StatsLog events.

Test: statsd_test

      and manual test, and run:

      adb shell cmd stats dump-report
      We have a default config, which contains a metrics to count PROCESS_START event sliced by
      package name.

Change-Id: I4838cc6cf025c143b7e84f43040703a78121fd25