Revert "Revert "Promote startActivityAsUser to SystemApi""

This reverts commit 07f7f3ff.

Currently, Context#startActivityAsUser(Intent, UserHandle) has
already been a System API. This CL furthur makes
startActivityAsUser with ActivityOptions a System API.
This is enssential to launch an Activity to the secondary display
by setting launched display ID in ActivityOptions.

This also align the behavior of startActivity. We made all versions
of startActivity public APIs.

Bug: 199829817
Change-Id: I334c2a98483e709559162ad64f3b69cbd3b615d2
Test: atest AnnotationTest
CTS-Coverage-Bug: 222189824