Fix separator keys for LayoutAnimation

Summary:
Should key separators with their cells.

**Test Plan**

before/after in this video: https://youtu.be/vid1w5x8-58

Reviewed By: achen1

Differential Revision: D4821708

fbshipit-source-id: 261f1bac34dfa9001297a24a44f11128f338e62b

Libraries/Lists/VirtualizedList.js

@@ -351,7 +351,7 @@ class VirtualizedList extends React.PureComponent<OptionalProps, Props, State> {
         />
       );
       if (ItemSeparatorComponent && ii < end) {
-        cells.push(<ItemSeparatorComponent key={'sep' + ii}/>);
+        cells.push(<ItemSeparatorComponent key={'sep' + key}/>);
       }
     }
   }