- 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