[native] introduce emoji avatar creation screen

Summary:
Introduce emoji avatar creation screen. Subsequent diffs will integrate this into the profile navigator/navigating to this screen. This diff is purely just a UI/UX diff for the avatar creation screen, and the next diff will handle the behavior/UX when a user tries to save their new emoji avatar

Link to figma

Design:

Depends on D7373

Test Plan:
Please watch the demo video to see how the emoji avatar creation screen works.

UPDATE:
Got some feedback regarding the colors we were using, synced with Ted and now this is what the screen will look like visually:

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7323