fix: onTabChange called twice on Android (ptomasroos#1093)

`onPageSelected={this._updateSelectedPage}` ViewPager prop causes onTabChange to be called twice when change tabs without any gesture actions
this will fix duplicated tab updates on android

#### when user clicked tab buttons
`goToPage -> updateSceneKeys(onTabChange) -> Screen Update -> onPageSelected={_updateSelectedPage} -> updateSceneKeys(onTabChange) -> Screen Update`

#### when user swiped tab views
`Swipe -> onPageSelected={_updateSelectedPage} -> updateSceneKeys(onTabChange) -> Screen Update`

#### when user clicked tab buttons with this patch
`goToPage -> set tabWillChangeWithoutGesture -> updateSceneKeys(onTabChange) -> Screen Update -> onPageSelected={_updateSelectedPage} -> ignore updateSceneKeys(onTabChange) -> remove tabWillChangeWithoutGesture`

Author: bang9

index.js

@@ -32,6 +32,7 @@ const ScrollableTabView = createReactClass({
     ScrollableTabBar,
   },
   scrollOnMountCalled: false,
+  tabWillChangeWithoutGesture: false,
 
   propTypes: {
     tabBarPosition: PropTypes.oneOf(['top', 'bottom', 'overlayTop', 'overlayBottom', ]),
@@ -141,6 +142,7 @@ const ScrollableTabView = createReactClass({
       }
     } else {
       if (this.scrollView) {
+        this.tabWillChangeWithoutGesture = true;
         if (this.props.scrollWithoutAnimation) {
           this.scrollView.setPageWithoutAnimation(pageNumber);
         } else {
@@ -305,10 +307,11 @@ const ScrollableTabView = createReactClass({
     }
 
     const currentPage = this.state.currentPage;
-    this.updateSceneKeys({
+    !this.tabWillChangeWithoutGesture && this.updateSceneKeys({
       page: localNextPage,
       callback: this._onChangeTab.bind(this, currentPage, localNextPage),
     });
+    this.tabWillChangeWithoutGesture = false;
   },
 
   _onChangeTab(prevPage, currentPage) {