[lib] Create notifs Olm session

Summary:
Use the keyserver keys to create the sessions.

Depends on D10666

Test Plan: Get access token, set it, and check if the session is created correctly... At this point it isn't because of invalid signature, probably due to prekey format - I'm investigating it.

Reviewers: varun, marcin, inka, kamil

Reviewed By: marcin, kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10667