[lib] Update messageStore.messages with msgStoreOps for updateMultimediaMessageMedia

Summary:
Instead of using messageStore.messages from the previous updatedMessageStore object, we pull messageStore.messages from processedMessageStore.messages instead.

Depends on D4204

Test Plan: This should be a straightforward refactor given we've had assertMessageStoreMessagesAreEqual in place for some time now.

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4205