fix removing views from ViewPagerAndroid

Summary:
Two things in this diff:

1. Implemented `getItemPosition` in our adapter; the default implementation always returns POSITION_UNCHANGED, which is incorrect, and causes `destroyItem` to never (sometimes?) be called.
2. Fix `destroyItem`: this never worked. `destroyItem` is always called by the ViewPager after a `notifyDataSetChanged()`, so after `removeViewAt`, which removes the view from `mViews`, causing `destroyItem` to throw `IndexOutOfBoundsException` when it tries to get the view. Since our item objects are just views, use that instead of checking `mViews`.

Reviewed By: ahmedre

Differential Revision: D3555427

fbshipit-source-id: 900c2696162d07f507e850517d483b943ce39a35

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

@@ -33,7 +33,7 @@
 
   private class Adapter extends PagerAdapter {
 
-    private List<View> mViews = new ArrayList<>();
+    private final List<View> mViews = new ArrayList<>();
 
     void addView(View child, int index) {
       mViews.add(index, child);
@@ -67,6 +67,11 @@ public int getCount() {
       return mViews.size();
     }
 
+    @Override
+    public int getItemPosition(Object object) {
+      return mViews.contains(object) ? mViews.indexOf(object) : POSITION_NONE;
+    }
+
     @Override
     public Object instantiateItem(ViewGroup container, int position) {
       View view = mViews.get(position);
@@ -76,8 +81,7 @@ public Object instantiateItem(ViewGroup container, int position) {
 
     @Override
     public void destroyItem(ViewGroup container, int position, Object object) {
-      View view = mViews.get(position);
-      container.removeView(view);
+      container.removeView((View) object);
     }
 
     @Override