Merge "Add and use run_as::run_as_child_app helper" into main am: 326a4c8fad

Original change: https://android-review.googlesource.com/c/platform/system/security/+/3318119

Change-Id: I67467e1ee191ff4048a7a33c902416551b77231b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>