Change Stack to have generic WindowContainer children (76/n)

- Step towards Tasks containing other Tasks.
- There are several places where we want to perform an operation on all
tasks in a branch of the hierarchy. We end up looping based on the
parent knowing what type of children is in the sub-branch. Instead use
forAllTasks() to perfrom the operation on all tasks in the sub-branch.

Bug: 80414790
Test: Existing tests pass
Change-Id: I5551fe440e56bb8d6982002f93037827b00c70fc