[native] introduce useSaveThreadAvatar hook

Summary:
Similar logic to D7424 but for thread avatars

Depends on D7426

Test Plan:
Able to save a thread avatar and double checked that the avatar column for the correct thread in the threads table was being populated

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7427