Avoid activity positioned on top of always-on-top activities

An activity was positioned on top of the Task while Task#addChild()
called when the activity was started, regardless there are other
always-on-top activities in the Task.

There isn't an issue because the activity will be positioned below
the always-on-top activity later when `task.positionChildAtTop(r)`
is called (after the activity added to the Task).

Bug: 181721787
Test: atest StartActivityTests#testStartActivitiesTaskOverlayStayOnTop
Change-Id: If166b866acd7aed4efce90a65b994b60bfa504c2