[lib][native] Initialize notif Olm channel during keyserver session recovery

Summary:
I missed adding this optional param, which was definitely an omission. Besides passing it in, this diff makes it a required param.

Depends on D11219

Test Plan:
In addition to the plan described here: https://gist.github.com/Ashoat/75ab690d5c53cdd68a51b02e03e27c58

I also made sure that after session recovery, than an entry in the olm_sessions MariaDB table existed for the new cookie with is_content = 0

Reviewers: marcin, inka, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D11220