StatsPullerManager not use singleton

This is to be consistent with other patterns such as UidMap.
This also makes unit test simpler.

Change-Id: I1558cd609e470481f269ecf2ae616277a95cfbf0
Bug: 72722120
Test: unit test
diff --git a/cmds/statsd/src/StatsService.h b/cmds/statsd/src/StatsService.h
index b3a4776..0a11f7c 100644
--- a/cmds/statsd/src/StatsService.h
+++ b/cmds/statsd/src/StatsService.h
@@ -246,9 +246,9 @@
     sp<UidMap> mUidMap;
 
     /**
-     * Fetches external metrics.
+     * Fetches external metrics
      */
-    StatsPullerManager mStatsPullerManager;
+    sp<StatsPullerManager> mPullerManager;
 
     /**
      * Tracks the configurations that have been passed to statsd.