[lib] Prefix blob holders with device ID

Summary:
Address ENG-9282.
Made multimedia blob holders prefixed by device ID.

Device ID is a base64-like string, which isn't a valid holder so used base64url conversion.

Test Plan: Successfully uploaded multimedia to a DM thread. Printed the

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13401