[lib] Unify duplicated avatar tcomb validators

Summary:
Noticed that we had two duplicated validators for avatar types. This diff unifies them, and opts for the more specific validator where there were differences (in just one case).

Depends on D8146

Test Plan: Flow

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8147