[lib] Update rawMessageInfoFromClientDB preconditions to allow MULTIMEDIA messages.

Summary:
invariant at start of rawMessageInfoFromClientDB currently always fails for messages of type MULTIMEDIA. This diff updates the invariant to accept either MULTIMEDIA or IMAGES messages.

Actual handling of MULTIMEDIA messages will come in a subsequent diff... this is mostly just to put up changes that can be landed as-is to reduce number of changes I have to juggle locally.

Test Plan: Things continue to work as expected.

Reviewers: tomek, marcin, ginsu, rohan, kamil, O2 Blocking Reviewers, ashoat

Reviewed By: O2 Blocking Reviewers, ashoat

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5636