[DO NOT MERGE] Check caller's uid in backupAgentCreated callback am: 859abb58cd am: d9702123cb am: 78e5fc0bd7

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/24308899

Change-Id: I74f9c411657c676c5b85ff313f7d52f7e361d009
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>