[commtest] Make ClientPublicKeys::device_id() return str slice

Summary:
Depends on D12341

Minor change for better reusability. It's better to have a slice and use to_string() only when needed.

Test Plan: Commtest

Reviewers: kamil, will, varun, marcin

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12342