[media] Remove stale MailboxHolder refs

Update stale MailboxHolder refs renaming them as they correspond
to shared images instead.

Bug: 383309406
Change-Id: Iebf6a39308b89b447b0b4d9d903f5094a8c78b2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7454135
Commit-Queue: Saifuddin Hitawala <hitawala@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Min Chen <minch@chromium.org>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1567927}
NOKEYCHECK=True
GitOrigin-RevId: d2c678a47f30c24403966b6c648cf381fc8daba9