[native] improve avatar component prop types

Summary:
Minor changes to improve the prop types of Avatar. Also made order of size from smallest to biggest (probably should have done this from the start) to improve readability

Depends on D7264

Test Plan: flow and all avatars still render

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7265