[automerge] Fix exposing private messages files through attachments with a content URI. 2p: 0d5452146c 2p: 471c3e9320

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

Bug: 275552292
Change-Id: I48af374ac45d21758789153013278d68290ee701