[keyserver] Introduce getMediaMessageServerDBContentsFromMedia(...)

Summary:
Pull out common logic from multimedia-message-spec:messageContentForServerDB(...) and native/input-state-container:sendMultimediaMessageAction(...) and introduce helper function.

This is just a straightforward refactor to cut redundancy.

Depends on D5197

Test Plan: Set breakpoints and verified that values were as expected at each step.

Reviewers: tomek, abosh, marcin

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5198