[lib] Fail signing device list if called by secondary device

Summary: Made signDeviceList() throw when called from non-primary devices.

Test Plan:

• Unit tests - needed to update them because otherwise mocked primaryContentSigningKey was different than primary device ID from device lists and some tests failed on the throw added in this diff - this proofs that it actually works.
• Tested manually

Reviewers: kamil, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13043