[lib] introduce getDefaultAvatar helper function

Summary: This logic will be used for both user and thread avatars so introduced getDefaultAvatar which is a helper function so I wouldn't need to copy/paste the logic in both places

Test Plan: flow and user avatars still render as expected with this new helper function. This diff will be further tested in subsequent diffs as well

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7131