Merge "Revert "Revert "Promote startActivityAsUser to SystemApi""" into tm-dev am: 2828207a am: 46a75a74

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/17076607

Change-Id: I3fd3530f4519b56813a9e934549d36798125bdd5