Fix CarDisplayTracker bugs

This change fixes the following:
- Prevents callbacks for other users from executing for the foreground
  user
- Prevents crashes from getOccupantZoneForUser returning null
- Updates the user for an occupant zone based on OccupantZoneManager
  callback
- Prevent WeakReference becoming null after check (causing an exception)

Bug: 267234924
Test: manual
Change-Id: Iaadff0576536de50280717fb08d58c298c98c87f