[commtest] Generate valid prekeys with mock Olm account

Summary:
Some tweaks to commtest mock Olm account:

• Added distinction between content and notif account
• Possibility to generate valid prekeys with signatures
• Updated prekey test to use the new functionality

Test Plan: commtest prekey test

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D14459