Add hidden API startActivityForResultAsUser

Bug: 37197930

Test: runtest -x cts/tests/app/src/android/app/cts/FragmentReceiveResultTest.java
      This verified existing startActivity* public APIs are still working
Test: The new hidden API is used/verified by the CLs under the same topic.

Change-Id: I2aca2823ecf26dc8a9318b3783b6041796c02582