Clean up how we handle configurations, and other assorted cleanup

- Add a ConfigManager class that tracks the configurations
  that have been passed to us.  Configurations are now
  tracked by tuples of (uid,tag), where the tag is an
  app-defined string, in case a single uid has multiple
  configurations.
- Move all of the initialization into StatsService.
- Get rid of the ability to have multiple LogListeners. Raw
  events are now pushed directly into StatsService, which
  can distribute them to the interested parties (and will
  eventually be able to do the proper locking).
- Add Log.h, which sets our LOG_TAG correctly.
- Move some of the related files that I expect will grow some
  into their own subdirectories.

Test: statsd_test
Test: adb shell cmd stats config ...
Test: adb shell dumpsys stats
Change-Id: I79487603003d8a842d5bd319741f1ecbf72063d1