[lib][native][web] Refactor useInitialNotificationsEncryptedMessage to get Olm session creator from context

Summary:
Now that we have a cross-platform context exposing this value, we can get it from there and simplify the API for useInitialNotificationsEncryptedMessage a little bit.

Depends on D11218

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: tomek, marcin

Reviewed By: marcin

Differential Revision: https://phab.comm.dev/D11219