[lib] Consolidate baseUpdateImageUserAvatar into baseSetUserAvatar

Summary:
After removing all the platform-specific stuff from baseUpdateImageUserAvatar and baseSetUserAvatar, there was one line different between them. Seemed simpler to handle with a conditional than two separate functions. The less code the better?

Depends on D8360

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

Reviewers: ashoat, ginsu, rohan

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8363