[lib] rename NotificationsSessionCreatorContext -> OlmSessionCreatorContext

Summary:
We now need to support creating content sessions in the same context - it's more readable than creating separate ones, as those will be very similar.

Depends on D10750

Test Plan: FLow

Reviewers: tomek, marcin

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10751