Ensure the bubble task is a root task

The app task was initially in Desktop mode and was then moved to
background. The next time starting the app into bubble, a trampoline
activity started in a new Task#A. Then, the trampoline starts the
main activity (and finish itself), which brings the existing Task#B
to front. In this case, the Task#B was still has a parent Desk task.

There are some issues in this case:
1. Exceptions thrown when the Task#B is set to always-on-top
2. The desk handler request transitions that caused flickers while
   animating open-bubble.
3. The other app tasks that were also in the Desktop mode becomes
   visible.

This CL ensure that the bubble task is the root task in order to
prevent other unrelated (i.e. Desk root task) being collected in
the transition.

Note that this is a temporary solution. Once the bubbles are
organized by a created-by-organizer root-task, the Task#B should
be reparented to the bubble root-task when it was started by the
trampoline.

Bug: 430687520
Test: start an app to Desktop, to background, start it again to bubble
Flag: EXEMPT bugfix

Change-Id: I0e5d01d98f4f8cc406b9077cd5848fe8486d7c99