[lib] Migrate Multimedia notifText to use EntityText

Summary: Depends on D6583

Test Plan: Generated Multimedia message on a web client in a chat with a different test user that is logged in on the iOS simulator. Background the iOS app and make sure the notif looks right

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6584