[native] Change AvatarSelection button to say "Submit" instead of "Next"

Summary:
Realized the final screen is supposed to say "Submit" in the designs.

Depends on D8084

Test Plan: Make sure the button says "Submit" now and matches designs

Reviewers: ginsu, atul

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8085