Added non-app window tokens to window container hierarchy

WindowContainer only allows its sub-classes to have homogenerous
children, however DisplayContent needs to have app window containers
that are managed by TaskStack and non-app window containers that are
managed by WindowToken. To do this we create a pass through class
DisplayChildWindowContainer that any container that wants to be a child
of DisplayContent needs to inherit from. Then, we can have DisplayContent
contain DisplayChildWindowContainer types and have sub-class children
like TaskStackContainers and NonAppWindowContainers

Bug: 30060889
Test: Existing tests pass.
Change-Id: I3848958eddcd51030cfe20bb1337476d203bbed3