[lib] Move invariant in changeThreadSettings above callServerEndpoint

Summary: Unless I'm mistaken, it makes more sense to check this before we make a call to the keyserver?

Test Plan: Things continue to work as expected, we don't expect to see this invariant hit.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7305