Rename native event timeStamp to timestamp

Summary: It's called `timestamp` on iOS, making it consistent.

Reviewed By: foghina

Differential Revision: D3820937

fbshipit-source-id: 2805f1fc10d6445d8b31676e0e3dca348510ffe7

ReactAndroid/src/test/java/com/facebook/react/RootViewTest.java

@@ -149,7 +149,7 @@ public class RootViewTest {
             0.,
             "target",
             rootViewId,
-            "timeStamp",
+            "timestamp",
             (double) ts,
             "identifier",
             0.));
@@ -186,7 +186,7 @@ public class RootViewTest {
             0.,
             "target",
             rootViewId,
-            "timeStamp",
+            "timestamp",
             (double) ts,
             "identifier",
             0.));