Touch events should have same timestmap as native Android MotionEvent

Summary:
When TouchEvent is created in RN, we're currently using System.currentTimeMillis - but this can differ from the MotionEvent timestamp by a few milliseconds.

This difference is very minor but makes it challenging to implement touch telemetry. It's easy and should be zero-impact otherwise to align the timestamps.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D31183733

fbshipit-source-id: 5b275ee534658dc429beb1d3cec0c83a779b5ea3

ReactAndroid/src/main/java/com/facebook/react/uimanager/events/Event.java

@@ -56,8 +56,16 @@ public abstract class Event<T extends Event> {
     init(-1, viewTag);
   }
 
-  /** This method needs to be called before event is sent to event dispatcher. */
   protected void init(int surfaceId, int viewTag) {
+    init(surfaceId, viewTag, SystemClock.uptimeMillis());
+  }
+
+  /**
+   * This method needs to be called before event is sent to event dispatcher. Event timestamps can
+   * optionally be dated/backdated to a custom time: for example, touch events should be dated with
+   * the system event time.
+   */
+  protected void init(int surfaceId, int viewTag, long timestampMs) {
     mSurfaceId = surfaceId;
     mViewTag = viewTag;
 
@@ -73,7 +81,7 @@ public abstract class Event<T extends Event> {
     // At some point it would be great to pass the SurfaceContext here instead.
     mUIManagerType = (surfaceId == -1 ? UIManagerType.DEFAULT : UIManagerType.FABRIC);
 
-    mTimestampMs = SystemClock.uptimeMillis();
+    mTimestampMs = timestampMs;
     mInitialized = true;
   }
 

ReactAndroid/src/main/java/com/facebook/react/uimanager/events/TouchEvent.java

@@ -96,7 +96,7 @@ public class TouchEvent extends Event<TouchEvent> {
       float viewX,
       float viewY,
       TouchEventCoalescingKeyHelper touchEventCoalescingKeyHelper) {
-    super.init(surfaceId, viewTag);
+    super.init(surfaceId, viewTag, motionEventToCopy.getEventTime());
 
     SoftAssertions.assertCondition(
         gestureStartTime != UNSET, "Gesture start time must be initialized");