Unit tests for ValueMetricProducer
StatsPullerManager is refactored so that we can mock it.
It may need more refactor pass to make is safer for longer runs.

Test: unit test
Change-Id: Ief0c99710e4d06e1454678f8b749c9599467d114
diff --git a/cmds/statsd/src/StatsService.h b/cmds/statsd/src/StatsService.h
index 1d7e5a61..fa92f65 100644
--- a/cmds/statsd/src/StatsService.h
+++ b/cmds/statsd/src/StatsService.h
@@ -157,7 +157,7 @@
     /**
      * Fetches external metrics.
      */
-    StatsPullerManager& mStatsPullerManager = StatsPullerManager::GetInstance();
+    StatsPullerManager mStatsPullerManager;
 
     /**
      * Tracks the configurations that have been passed to statsd.