Use pending transition when task-org is unregistered

When a task-org is unregistered, the surface controls of all the
tasks organized by it are recreated, and their parent-child
relationship is reestablished with the new surface controls.

However, as different transactions are used for this recreate
operations, reparent() could be applied in an unexpected order,
and could some layers left detached in SF.

This CL replaces sync transition with pending transition in
TaskOrgController#removeTask() to ensure that the same transaction
is used for a series of operations.

Bug: 271357562
Test: WM CTS no longer causes ANR.
Change-Id: I8a95d3f03ae8f5dfafbca29b7e1bd252762c351c