[native] Make callback param optional in useUploadSelectedMedia avatar hook

Summary:
We don't need this callback param in the registration flow, since we're already wrapping the call to useRegistrationServerCall with some loading state.

Depends on D8149

Test Plan: Flow

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8150