Support legacy event emitter

Summary:
Changelog: [Internal] - Add support for legacy event emitter for PointerEvents because it's used by natively driven Animated events

It works by
* [NativeAnimatedNodesManager being a listener on every event dispatched](https://www.internalfb.com/code/fbsource/[b8fc8603f8d84003bcb73ec84e8490c136df290c]/xplat/js/react-native-github/ReactAndroid/src/main/java/com/facebook/react/uimanager/events/EventDispatcherImpl.java?lines=115-117)
* NativeAnimatedNodesManager [re-dispatching the event onto an EventAnimationDriver to update the animated value.](https://www.internalfb.com/code/fbsource/[d5403bc1fa55a68d9346ac693378551b734c565b]/xplat/js/react-native-github/ReactAndroid/src/main/java/com/facebook/react/animated/NativeAnimatedNodesManager.java?lines=597)

As such, we need to make sure to implement the legacy event dispatch function.

Reviewed By: vincentriemer

Differential Revision: D37769785

fbshipit-source-id: c570896afc624193fdb636a302e9613ca6fee14d

Author: Luna Wei

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

@@ -72,6 +72,7 @@ public static PointerEvent obtain(
   private int mCoalescingKey = UNSET_COALESCING_KEY;
   private float mOffsetX;
   private float mOffsetY;
+  private @Nullable List<WritableMap> mPointersEventData;
 
   private void init(
       String eventName,
@@ -98,12 +99,33 @@ public String getEventName() {
 
   @Override
   public void dispatch(RCTEventEmitter rctEventEmitter) {
-    // Skip legacy stuff for now?
+    if (mMotionEvent == null) {
+      ReactSoftExceptionLogger.logSoftException(
+          TAG,
+          new IllegalStateException(
+              "Cannot dispatch a Pointer that has no MotionEvent; the PointerEvehas been recycled"));
+      return;
+    }
+    if (mPointersEventData == null) {
+      mPointersEventData = createPointersEventData();
+    }
+
+    if (mPointersEventData == null) {
+      // No relevant MotionEvent to dispatch
+      return;
+    }
+
+    boolean shouldCopy = mPointersEventData.size() > 1;
+    for (WritableMap pointerEventData : mPointersEventData) {
+      WritableMap eventData = shouldCopy ? pointerEventData.copy() : pointerEventData;
+      rctEventEmitter.receiveEvent(this.getViewTag(), mEventName, eventData);
+    }
     return;
   }
 
   @Override
   public void onDispose() {
+    mPointersEventData = null;
     MotionEvent motionEvent = mMotionEvent;
     mMotionEvent = null;
     if (motionEvent != null) {
@@ -130,13 +152,13 @@ private ArrayList<WritableMap> createPointerEvents() {
     ArrayList<WritableMap> pointerEvents = new ArrayList<>();
 
     for (int index = 0; index < motionEvent.getPointerCount(); index++) {
-      pointerEvents.add(this.createPointerEvent(index));
+      pointerEvents.add(this.createPointerEventData(index));
     }
 
     return pointerEvents;
   }
 
-  private WritableMap createPointerEvent(int index) {
+  private WritableMap createPointerEventData(int index) {
     WritableMap pointerEvent = Arguments.createMap();
 
     pointerEvent.putDouble("pointerId", mMotionEvent.getPointerId(index));
@@ -158,41 +180,48 @@ private WritableMap createPointerEvent(int index) {
     return pointerEvent;
   }
 
-  @Override
-  public void dispatchModern(RCTModernEventEmitter rctEventEmitter) {
-    if (mMotionEvent == null) {
-      ReactSoftExceptionLogger.logSoftException(
-          TAG,
-          new IllegalStateException(
-              "Cannot dispatch a Pointer that has no MotionEvent; the PointerEvehas been recycled"));
-      return;
-    }
-
-    List<WritableMap> relevantPointerEventData = null;
-
+  private List<WritableMap> createPointersEventData() {
     int activePointerIndex = mMotionEvent.getActionIndex();
+    List<WritableMap> pointersEventData = null;
     switch (mEventName) {
         // Cases where all pointer info is relevant
       case PointerEventHelper.POINTER_MOVE:
       case PointerEventHelper.POINTER_CANCEL:
-        relevantPointerEventData = createPointerEvents();
+        pointersEventData = createPointerEvents();
         break;
         // Cases where only the "active" pointer info is relevant
       case PointerEventHelper.POINTER_ENTER:
       case PointerEventHelper.POINTER_DOWN:
       case PointerEventHelper.POINTER_UP:
       case PointerEventHelper.POINTER_LEAVE:
-        relevantPointerEventData = Arrays.asList(createPointerEvent(activePointerIndex));
+        pointersEventData = Arrays.asList(createPointerEventData(activePointerIndex));
         break;
     }
 
-    if (relevantPointerEventData == null) {
+    return pointersEventData;
+  }
+
+  @Override
+  public void dispatchModern(RCTModernEventEmitter rctEventEmitter) {
+    if (mMotionEvent == null) {
+      ReactSoftExceptionLogger.logSoftException(
+          TAG,
+          new IllegalStateException(
+              "Cannot dispatch a Pointer that has no MotionEvent; the PointerEvehas been recycled"));
+      return;
+    }
+
+    if (mPointersEventData == null) {
+      mPointersEventData = createPointersEventData();
+    }
+
+    if (mPointersEventData == null) {
       // No relevant MotionEvent to dispatch
       return;
     }
 
-    boolean shouldCopy = relevantPointerEventData.size() > 1;
-    for (WritableMap pointerEventData : relevantPointerEventData) {
+    boolean shouldCopy = mPointersEventData.size() > 1;
+    for (WritableMap pointerEventData : mPointersEventData) {
       WritableMap eventData = shouldCopy ? pointerEventData.copy() : pointerEventData;
       rctEventEmitter.receiveEvent(
           this.getSurfaceId(),