[lib] introduce savedEmojiAvatarSelectorForCurrentUser selector

Summary:
Based on some feedback I got in D7359, when we "reset" the emoji avatar during the emoji avatar creation flow, we want to reset to the current state in redux which we are considering the source of truth. This is the selector that would be called in EmojiAvatarCreation to get the current emoji avatar from redux for the current user

Depends on D7401

Test Plan:
The reset functionality behaves as expected as seen in the demo below

An example where we are getting a random default avatar:

Reviewers: atul, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7402