Rename Stack to Task related terms (3/n)

Update most of methods and members with Stack terms in below files
- ActivityTaskManagerService
- RootWindowContainer
- ActivityTaskSupervisor
- DisplayContent
- TaskDisplayArea

No logical changes.

Bug: 157876447
Test: presubmit
Change-Id: I333c29916fa34fbccf9a7e172b79689b422259a3