Fix warning in ScrollView classes

Summary:
EZ diff to fix warning in ScrollView classes

changelog: [internal] internal

Reviewed By: JoshuaGross

Differential Revision: D31175085

fbshipit-source-id: 2aebcf6b53b82f8d96aadf40db3f459652c69fb2

ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollViewHelper.java

@@ -40,7 +40,7 @@ public class ReactScrollViewHelper {
   }
 
   // Support global native listeners for scroll events
-  private static Set<ScrollListener> sScrollListeners =
+  private static final Set<ScrollListener> sScrollListeners =
       Collections.newSetFromMap(new WeakHashMap<ScrollListener, Boolean>());
 
   // If all else fails, this is the hardcoded value in OverScroller.java, in AOSP.