Add onPageScrollStateChanged for ViewPagerAndroid

Summary:
I have an issue when combining `PullToRefreshViewAndroid` and `ViewPagerAndroid`.
`ViewPagerAndroid` will not able to scroll that gesture handler is being taken by `PullToRefreshViewAndroid`
One solution is to disable `PullToRefreshViewAndroid` if `ViewPagerAndroid` is scrolling (i.e. not idle).
[Reference solution here](http://stackoverflow.com/a/29946734/2590265)

So here need to expose the `onPageScrollStateChanged` event.
Some code referenced from  DrawerLayoutAndroid, especially the `VIEWPAGER_PAGE_SCROLL_STATES` array.
Please feel free give me comments.
Thanks.
Closes https://github.com/facebook/react-native/pull/5026

Reviewed By: svcscm

Differential Revision: D2830623

Pulled By: andreicoman11

fb-gh-sync-id: c2a6920c6f4c7daab0115f13864db83b93b31abf

ReactAndroid/src/main/java/com/facebook/react/views/viewpager/PageScrollStateChangedEvent.java

@@ -0,0 +1,49 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+package com.facebook.react.views.viewpager;
+
+import com.facebook.react.bridge.Arguments;
+import com.facebook.react.bridge.WritableMap;
+import com.facebook.react.uimanager.events.Event;
+import com.facebook.react.uimanager.events.RCTEventEmitter;
+
+/**
+ * Event emitted by {@link ReactViewPager} when user scrolling state changed.
+ *
+ * Additional data provided by this event:
+ *  - pageScrollState - {Idle,Dragging,Settling}
+ */
+class PageScrollStateChangedEvent extends Event<PageScrollStateChangedEvent> {
+
+  public static final String EVENT_NAME = "topPageScrollStateChanged";
+
+  private final String mPageScrollState;
+
+  protected PageScrollStateChangedEvent(int viewTag, long timestampMs, String pageScrollState) {
+    super(viewTag, timestampMs);
+    mPageScrollState = pageScrollState;
+  }
+
+  @Override
+  public String getEventName() {
+    return EVENT_NAME;
+  }
+
+  @Override
+  public void dispatch(RCTEventEmitter rctEventEmitter) {
+    rctEventEmitter.receiveEvent(getViewTag(), getEventName(), serializeEventData());
+  }
+
+  private WritableMap serializeEventData() {
+    WritableMap eventData = Arguments.createMap();
+    eventData.putString("pageScrollState", mPageScrollState);
+    return eventData;
+  }
+}

ReactAndroid/src/main/java/com/facebook/react/views/viewpager/ReactViewPager.java

@@ -104,7 +104,22 @@ import com.facebook.react.uimanager.events.NativeGestureUtil;
 
     @Override
     public void onPageScrollStateChanged(int state) {
-      // don't send events
+      String pageScrollState;
+      switch (state) {
+        case SCROLL_STATE_IDLE:
+          pageScrollState = "idle";
+          break;
+        case SCROLL_STATE_DRAGGING:
+          pageScrollState = "dragging";
+          break;
+        case SCROLL_STATE_SETTLING:
+          pageScrollState = "settling";
+          break;
+        default:
+          throw new IllegalStateException("Unsupported pageScrollState");
+      }
+      mEventDispatcher.dispatchEvent(
+        new PageScrollStateChangedEvent(getId(), SystemClock.uptimeMillis(), pageScrollState));
     }
   }
 

ReactAndroid/src/main/java/com/facebook/react/views/viewpager/ReactViewPagerManager.java

@@ -50,6 +50,7 @@ public class ReactViewPagerManager extends ViewGroupManager<ReactViewPager> {
   public Map getExportedCustomDirectEventTypeConstants() {
     return MapBuilder.of(
         PageScrollEvent.EVENT_NAME, MapBuilder.of("registrationName", "onPageScroll"),
+        PageScrollStateChangedEvent.EVENT_NAME, MapBuilder.of("registrationName", "onPageScrollStateChanged"),
         PageSelectedEvent.EVENT_NAME, MapBuilder.of("registrationName", "onPageSelected")
     );
   }