ScrollViewStickyHeader should be exported from react-native

[tanishqkancharla]

Introduction

ScrollViewStickyHeader is a complex component capturing a lot of internal details to make sticky headers work on react-native's lists. It is not exported from react-native today, making properly implementing it a huge pain. If it was exported, we could use it directly in our implementations as well as give it custom styling.

Without it being exported, the StickyHeaderComponent prop in all the virtualized lists is extremely difficult to implement correctly.

Discussion points

I propose:

• Export ScrollViewStickyHeader
• Add .d.ts support for the StickyHeaderComponent prop on the virtualized lists (currently the props are typed as any)