[native] introduce EmojiSelection type

Summary: Created a new type called EmojiSelection based on this type found in the rn-emoji-keyboard codebase. This should be an improvement to what we had before ({ emoji: string, ... })

Test Plan: Everything still works and when I log out emoji in onEmojiSelected in native/avatars/emoji-avatar-creation.react.js I now get the improved field suggestions for emoji

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8227