Add square brackets around tags in logs in FabricUIManager

Summary:
See title.

Changelog: [Internal]

Differential Revision: D23257809

fbshipit-source-id: b8e519971603506e8ae2b327582d3e3e7d65fddf

ReactAndroid/src/main/java/com/facebook/react/fabric/mounting/mountitems/InsertMountItem.java

@@ -39,9 +39,9 @@ public class InsertMountItem implements MountItem {
   public String toString() {
     return "InsertMountItem ["
         + mReactTag
-        + "] - parentTag: "
+        + "] - parentTag: ["
         + mParentReactTag
-        + " - index: "
+        + "] - index: "
         + mIndex;
   }
 }