[automerge] Fix exposing private messages files through attachments with a content URI. 2p: 0d5452146c

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/Messaging/+/23089831

Bug: 275552292
Change-Id: I70bfdb81846c81cf0c79d9a1f33140e00734f163