Memoize VirtualizedListCellContextProvider
Memoize VirtualizedListCellContextProvider

Summary:
When a FlatList is nested inside another FlatList, it may be re-rendered whenever the outer FlatList renders. Apply the same optimization we already had in `VirtualizedListContextProvider` to avoid changing the context object if no values have changed.

Changelog: [General][Changed] - Optimized VirtualizedList context when used with nested lists

Reviewed By: genkikondo

Differential Revision: D35905952

fbshipit-source-id: 695253c85db2043d22e208ad94ecc7daa1455055

facebook/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,javache/react-native,facebook/react-native,janicduplessis/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native,facebook/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native,janicduplessis/react-native,javache/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native,facebook/react-native