- 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
