metricsd: Split into two daemons.

This CL splits metrics_daemon into two independent daemons: metricsd and
metrics_collector.

* metricsd will be responsible for reading the metrics from disk and
  periodically uploading them to the server.
* metrics_collector will be responsible for gathering generic metrics
  from the system and managing the weave state for metrics.

This refactoring is necessary to prepare the migration of metricsd to
log over binder.

Bug: 25670908
Test: Unit tests.
Test: manual: both daemons run. Metrics are logged, uploaded and the
      weave interaction works.

Change-Id: Ib00e1772bb7eec87cbcdcd912c30b555d79d7074