Promote startActivityAsUser to SystemApi

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
Test: presubit
CTS-Coverage-Bug: 222189824
Change-Id: Ia1db353d4b91a8952d4d21b970253e1ac03e8173