Merge "[Multiple Users] Start activity as system user." into tm-dev am: 2c152dd1

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/17592026



Change-Id: I5589a90ac987bfbdb7c3f1eb027da068016a2346
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>