[native] Move complexity of actionSheetConfig to useShowAvatarActionSheet

Summary:
We can simplify the actionSheetConfig constructed in Edit*Avatar by moving some of what's common to useShowAvatarActionSheet. This cuts down on reptition a decent amount.

All of this refactoring brought the size of Edit*Avatar from 234 lines to 40:

This should make the subsequent changes easier to manage.

Depends on D7506

Test Plan: Everything looks and works as it did previously + ESLint + Flow

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7507