[native] introduce bottom sheet backdrop component

Summary:
By default the react native bottom sheet does has the backdropComponent set to null. In order to set the backdrop we need to create a backdrop component and pass it into the backdropComponent prop of BottomSheetModal

Depends on D8958

Test Plan:
Please see the demo video below to see how the backdrop works

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ted, ashoat, tomek

Differential Revision: https://phab.comm.dev/D8981