PointerEvents: Reset childHandlingNativeGesture on first ACTION_DOWN

Summary:
Changelog: [Internal] - Reset the mChildHandlingNativeGesture on the first ACTION_DOWN after it's been set.

I noticed this issue when scrolling sometimes in the event log for the W3CPointerEvents example, none of the pointer events would get captured because we'd call [`notifyNativeGestureStarted`](https://www.internalfb.com/code/fbsource/xplat/js/react-native-github/ReactAndroid/src/main/java/com/facebook/react/uimanager/events/NativeGestureUtil.java?lines=25-25) which would prevent any pointerevents being fired.

Reviewed By: vincentriemer

Differential Revision: D35685378

fbshipit-source-id: 01fc255afc5e22dc6c42f7eb11a8aa5a9a091b87

Author: Luna Wei

ReactAndroid/src/main/java/com/facebook/react/uimanager/JSPointerDispatcher.java

@@ -69,12 +69,6 @@ public void onChildEndedNativeGesture() {
   }
 
   public void handleMotionEvent(MotionEvent motionEvent, EventDispatcher eventDispatcher) {
-
-    // Ignore if child is handling native gesture
-    if (mChildHandlingNativeGesture != -1) {
-      return;
-    }
-
     boolean supportsHover =
         PointerEventHelper.supportsHover(motionEvent.getToolType(motionEvent.getActionIndex()));
 
@@ -101,6 +95,9 @@ public void handleMotionEvent(MotionEvent motionEvent, EventDispatcher eventDisp
     // First down pointer
     if (action == MotionEvent.ACTION_DOWN) {
 
+      // Reset mChildHandlingNativeGesture like JSTouchDispatcher does
+      mChildHandlingNativeGesture = -1;
+
       // Start a "down" coalescing key
       mDownStartTime = motionEvent.getEventTime();
       mTouchEventCoalescingKeyHelper.addCoalescingKey(mDownStartTime);
@@ -119,6 +116,12 @@ public void handleMotionEvent(MotionEvent motionEvent, EventDispatcher eventDisp
       return;
     }
 
+    // If the touch was intercepted by a child, we've already sent a cancel event to JS for this
+    // gesture, so we shouldn't send any more pointer events related to it.
+    if (mChildHandlingNativeGesture != -1) {
+      return;
+    }
+
     // New pointer goes down, this can only happen after ACTION_DOWN is sent for the first pointer
     if (action == MotionEvent.ACTION_POINTER_DOWN) {
       mTouchEventCoalescingKeyHelper.incrementCoalescingKey(mDownStartTime);