[native] Use object as param to getInitialNotificationsEncryptedMessage

Summary:
Right now this only takes a single parameter, but in the next diff I will introduce a second one.

Callsites will generally either use one or the other parameter, but no current callsites will use both. As such, I figured an object param would be "cleaner" since it would avoid the need for "skipped" params.

Test Plan: Flow

Reviewers: marcin, atul

Reviewed By: atul

Subscribers: tomek, wyilio

Differential Revision: https://phab.comm.dev/D9584