Experiment to replace Fabric MountItem lists with concurrent queues (re-land)

Summary:
Currently, queuing any sort of MountItem or getting the list of MountItems requires synchronization. Since these can be queued up at any point from the JS thread, there will, in theory, be constant contention between JS and UI thread.

To see if this is true, I'm creating an experiment instead of just switching over to concurrent structures. After seeing results, we can hopefully make a decision and delete the non-concurrent stuff or improve on it somehow.

The original was unlanded in D24973616 (https://github.com/facebook/react-native/commit/26787e2260412d9d2fe831e68a8616505d3cab36) due to a typo, which has been fixed now. The typo was that in Blocking Mode, we queued up all PreMountItems to the concurrent PreMountItems queue.

Changelog: [Internal]

Reviewed By: ShikaSD

Differential Revision: D24974851

fbshipit-source-id: d081c081bbd0de445bb92408f0ec822056b905a5

ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java

@@ -88,4 +88,7 @@ public class ReactFeatureFlags {
 
   /** Potential bugfix for crashes caused by mutating the view hierarchy during onDraw. */
   public static boolean enableDrawMutationFix = true;
+
+  /** Use lock-free data structures for Fabric MountItems. */
+  public static boolean enableLockFreeMountInstructions = false;
 }