Ensure that the recent tasks are loaded for SysUI's current user.

- At the point where we preload tasks, the current user in the system can
  be different than the "current" user from SystemUI's perspective. Instead
  of using the CURRENT user to get the recent tasks, use SystemUI's current
  user, to ensure that the tasks fetched are the same user as the user that
  we update the OVERVIEW_LAST_STACK_ACTIVE_TIME for (even if it's not the
  actual current user).
- In addition, if user switch happens between preloadRawTasks() and
  preloadPlan(), ensure we are using the user which we loaded recent tasks
  for, for the same reason as above.

Bug: 62440327
Test: Switch between users and ensure that tasks are not lost

Change-Id: Ia7bbf7f4c4002f6082f6f668c15a8da705ff23eb