[native] introduce avatar constants

Summary:
This diff address @atul's brittle code concerns in D9376

Depends on D9382

Test Plan: confirmed that there were no regressions with the sizing of avatars

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio, atul

Differential Revision: https://phab.comm.dev/D9383