base

Start moving Tasks from DisplayContent to TaskStack

- Create new classes for Stacks on WindowManager.
- Stop using DisplayContent methods and members:
    addAppToken(),
    removeAppToken(),
    setAppTaskId(),
    removeTask(),
    mTaskIdToDisplayContents,
    mTaskIdToTask.
- Start using WindowManagerService.createTask().
- Establish hierarchy of references: AppWindowToken=>Task=>
TaskStack=>StackBox=>DisplayContent.
- Clean up StackBox, TaskStack, and Task.

Change-Id: I798990aa7966784d22f4a43822087d8bb0404dd6