[native] move navigate to emoji thread avatar creation function to be within edit thread avatar

Summary:
Small cleanup diff, I figured it would be better to have this logic within EditThreadAvatar

Depends on D7643

Test Plan: flow still able to navigate to the user emoji avatar creation screen

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7644