Fix exposing private messages files through attachments with a content URI. am: 0d5452146c

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

Change-Id: I56f3015298ed0b2acde72442b458e0b354f0e13d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>