Create a root task for bubble

This is the very first CL to use a root task to manage bubbles.
It simply sets the launch-root-task while launching the app
bubbles. Most of the CUJs won't work yet, and will be done in the
later CLs.

Bug: 407669465
Test: wm presubmit
Test: start an app into bubble
Flag: com.android.window.flags.root_task_for_bubble
Change-Id: I6bbfc0617c662bffb12b55dfe48e5b829a164cbb