[lib] Establish holders for DM multimedia

Summary:
When receiving DM multimedia message (inbound), we should establish holders for the media.
Sender (outbound) has them already established during media upload.

Depends on D13413

Test Plan:
Created DM thread with two users. Sent a photo and a video. No holders were established for sender.
Recipients established 3 holders: photo, video and video thumbnail.

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13415