[commtest] Rename create_device to register_user_device

Summary:
The create_device wasn't very accurate name. What the function does is:

• Register a new user
• Upload keys for its primary device

Also added a DeviceType optional argument to the function which will be needed late optional argument to the function which will be needed later.

Depends on D10404

Test Plan: Commtest builds, CommTest CI

Reviewers: kamil, michal, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10420