Fixes flicker while starting new task from a bubble

Starting a new Bubble from an existing Bubble will immediately hide
the existing Bubble since the new Bubble Task bounds are the same
as its parent root task. So the transition will contain two changes,
one for the new open task, the second change is for the other bubble
task (TO_BACK).

Keep the task surface in the TaskView or it will be reparented to
the transition root during the transition and showing out of the
TaskView.

Bug: 407669465
Test: start bubble from an existing bubble
Flag: com.android.window.flags.root_task_for_bubble
Change-Id: Idea8cddc232e7d47b6f9a31b49f53db3a0de3724