[lib] Fix return value of getSIWENonce

Summary:
Fix getSIWENonce to return the nonce (string) as expected instead of the object that's returned by callServerEndpoint('siwe_nonce').

Depends on D5964

Test Plan: Was able to log values and set breakpoints to ensure that things were as expected.

Reviewers: ashoat, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D5965