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

Libraries/Components/ViewPager/ViewPagerAndroid.android.js

@@ -19,6 +19,14 @@ var requireNativeComponent = require('requireNativeComponent');
 
 var VIEWPAGER_REF = 'viewPager';
 
+type Event = Object;
+
+export type ViewPagerScrollState = $Enum<{
+  idle: string;
+  dragging: string;
+  settling: string;
+}>;
+
 /**
  * Container that allows to flip left and right between child views. Each
  * child view of the `ViewPagerAndroid` will be treated as a separate page
@@ -78,6 +86,16 @@ var ViewPagerAndroid = React.createClass({
      */
     onPageScroll: ReactPropTypes.func,
 
+    /**
+     * Function called when the page scrolling state has changed.
+     * The page scrolling state can be in 3 states:
+     * - idle, meaning there is no interaction with the page scroller happening at the time
+     * - dragging, meaning there is currently an interaction with the page scroller
+     * - settling, meaning that there was an interaction with the page scroller, and the
+     *   page scroller is now finishing it's closing or opening animation
+     */
+    onPageScrollStateChanged: ReactPropTypes.func,
+
     /**
      * This callback will be called once ViewPager finish navigating to selected page
      * (when user swipes between pages). The `event.nativeEvent` object passed to this
@@ -147,6 +165,12 @@ var ViewPagerAndroid = React.createClass({
     }
   },
 
+  _onPageScrollStateChanged: function(e: Event) {
+    if (this.props.onPageScrollStateChanged) {
+      this.props.onPageScrollStateChanged(e.nativeEvent.pageScrollState);
+    }
+  },
+
   _onPageSelected: function(e: Event) {
     if (this.props.onPageSelected) {
       this.props.onPageSelected(e);
@@ -183,6 +207,7 @@ var ViewPagerAndroid = React.createClass({
         ref={VIEWPAGER_REF}
         style={this.props.style}
         onPageScroll={this._onPageScroll}
+        onPageScrollStateChanged={this._onPageScrollStateChanged}
         onPageSelected={this._onPageSelected}
         children={this._childrenWithOverridenStyle()}
       />