[lib] Consolidate updateImageThreadAvatar into baseSetThreadAvatar

Summary:
This is the thread avatar version of D8363.

After removing all of the platform-specific stuff from updateImageThreadAvatar and baseSetThreadAvatar, there was one line different between them. Seemed simpler to handle w/ a conditional than two separate functions.

Test Plan: Close reading, made sure spinner continued to behave as expected when updating image avatar on native.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8774