Pre-cleanup before unifying Task and Stack (82/n)

- Removed Task#getTaskStack() and Task#mStack. Changed callers to use getStack().
- Made ActivityStack#mDisplayId private and changed callers to use getDisplayId.
- Switch stack id assignment to use getNextTaskIdForUser() so there isn't dups. with the ids.
- Made ActivityRecord#mTaskOverlay private.

Bug: 80414790
Test: Existing tests pass.
Change-Id: I47716f1fdac4cab21eed5a69f91e765f070e7550