Move plugin stats to their own file

Summary: Really nothing interesting to see here , just moved pluginStats out of messageQueue, as it started there and kept growing

Reviewed By: nikoant

Differential Revision: D22257318

fbshipit-source-id: 26be7efb4629fcef1b14de96a2b60f17f7d76785

desktop/app/src/dispatcher/tracking.tsx

@@ -17,7 +17,7 @@ import Client from '../Client';
 import {
   getPluginBackgroundStats,
   resetPluginBackgroundStatsDelta,
-} from '../utils/messageQueue';
+} from '../utils/pluginStats';
 import {
   clearTimeline,
   TrackingEvent,