Reuse stack as task for single task stacks (90/n)

Stacks and tasks are now the same object instance, so for cases were we
have a 1to1 relationship between the stack and task, just go ahead and
use one stack/task object instead of having the extra level.
- Helps us tryout the task hierarchy at a different depth than 2
to see what bugs needs to be fixed.
- Makes it easier to switch "tiles" into the hierarchy.

Bug: 80414790
Test: Existing tests pass.
Change-Id: I9796a149f5ca689b7000819085d4ed8e156b618c