Remove starting window whenever the acitvity is stopped

The main app window will never finish drawing at this point so there
is nothing to trigger the removal of the starting window.

Also, set ActivityRecord.mStartingWindowShown to true for some cases
where we were telling WM to show starting window but not setting the
flag that would later be used for clean-up.

Bug: 26659857
Change-Id: I7a8582521853f1f95b77d8b08f4dd0cf778f8cbf