Change Task to have generic WindowContainer children (73/n)

- Step towards Tasks containing other Tasks.
- There are several places where we want to perform an operation on all
activities 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 forAllActivities() to perfrom the operation on all
activities in the sub-branch.

Bug: 80414790
Test: Existing tests pass
Change-Id: I3dd08784e82b3eceac9aacacd4d1f1bd85ef8c02