Handle the case where launch root is not attached

- Throw illegalstateException if the task that is being set as launch
  root task is not attached to the wm hierarchy - in other words:
  when it doesn't have a display area.
- This makes sure that system server doesn't crash with NPE when this
  happens.

Bug: 283152174
Test: Tested manually using the car portrait target
Change-Id: I330560cbb1ecf8f267c37e1696084b6872c3068e