[lib] Make text and media message types read-only

Summary: Was updating these types and figured it would be good to make them read-only.

Test Plan:
Flow. Also I tested this whole stack as follows:

1. Create a sidebar by sending a text message on web
2. Create a sidebar by sending a media message on web
3. Create a sidebar by sending a text message on native
4. Create a sidebar by sending a media message on native

In all of these cases, I confirmed that only one notif was delivered (the notif for the text/media message was skipped)

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6795