Fix Modal + FlatList scrolling
Fix Modal + FlatList scrolling

Summary:
`FlatList` (actually `VirtualizedList`) allows recursive nesting of itself for easy and complex composition of lists such that it can support seemless virtualization and VPV events. It does this by only rendering a `ScrollView` for the outermost `VirtualizedList` and simply stacking `View`s for all the internal ones.

However, if a `Modal` is in a `FlatList` and also hosts a `FlatList`, e.g.:
```
<FlatList ListFooterComponent={<Modal><Foo /></Modal>} />
```
Then React context will propogate through to the inner `FlatList` and cause it to render as a plain `View`. Because the `Modal` actually portals the views into a different native hierarchy, one without a `ScrollView`, the `FlatList` won't scroll as expected.

The fix is to wipe out the context - not sure if there is a better way, but this doesn't seem terrible.

Differential Revision: D7863625

fbshipit-source-id: 38f41d72ed32b9f0eb1c9c82893f21d83a83f9ad

exponent/react-native,facebook/react-native,myntra/react-native,arthuralee/react-native,hoangpham95/react-native,javache/react-native,exponentjs/react-native,pandiaraj44/react-native,arthuralee/react-native,exponent/react-native,hammerandchisel/react-native,pandiaraj44/react-native,arthuralee/react-native,myntra/react-native,javache/react-native,hammerandchisel/react-native,myntra/react-native,hoangpham95/react-native,facebook/react-native,pandiaraj44/react-native,exponent/react-native,myntra/react-native,pandiaraj44/react-native,exponentjs/react-native,hammerandchisel/react-native,javache/react-native,hammerandchisel/react-native,javache/react-native,facebook/react-native,myntra/react-native,facebook/react-native,janicduplessis/react-native,hammerandchisel/react-native,exponent/react-native,javache/react-native,hammerandchisel/react-native,janicduplessis/react-native,hoangpham95/react-native,pandiaraj44/react-native,hammerandchisel/react-native,janicduplessis/react-native,hoangpham95/react-native,facebook/react-native,exponent/react-native,exponentjs/react-native,facebook/react-native,pandiaraj44/react-native,exponent/react-native,exponentjs/react-native,exponentjs/react-native,myntra/react-native,facebook/react-native,hoangpham95/react-native,hoangpham95/react-native,myntra/react-native,facebook/react-native,exponentjs/react-native,facebook/react-native,janicduplessis/react-native,hoangpham95/react-native,myntra/react-native,janicduplessis/react-native,exponentjs/react-native,arthuralee/react-native,hammerandchisel/react-native,pandiaraj44/react-native,javache/react-native,exponentjs/react-native,javache/react-native,arthuralee/react-native,javache/react-native,janicduplessis/react-native,myntra/react-native,exponent/react-native,janicduplessis/react-native,pandiaraj44/react-native,janicduplessis/react-native,hoangpham95/react-native,exponent/react-native,javache/react-native