[lib] introduce getRandomDefaultEmojiAvatar function

Summary: small utiltity function that will be used in the following diff

Test Plan: Tested in a subsequent diff locally that I was getting a random default avatar on return when calling the function

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7401