[lib] introduce savedEmojiAvatarSelectorForThread selector

Summary:
Similar logic to D7402 but for thread avatars. In addition to creating the selector, I needed to create a small utility function: getAvatarForThread which factors out most of the logic in useAvatarForThread but now we can easily use this logic in the savedEmojiAvatarSelectorForThread selector

Depends on D7424

Test Plan:
All thread avatars still render properly (shows that getAvatarForThread works as expected) , and the reset functionality for thread avatars behaves as expected:

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7426