Add a DropboxWriter in statsd.

+ The DropboxWriter keeps data in cache, and flush to files once the
  size exceeds the maximum value.

+ Different components should create their owner DropboxWriter with
  different tags, e.g., anomly detection, experiment metrics, etc.

+ Copied stats_log related protos from g3

Test: run statsd, and adb shell dumpsys dropbox
      Will add unit tests.

Change-Id: If06e9a9953be32082252b340a97124d732656b40