SlimRecents: Rework show top most task feature

recently introduced this feature needed a bit rework. Basic changes

- do not call settings provider on every preload to prevent high IO work.
  Use our observer for that
- Use internal flag logic to mark topmost task
- Move interpretation of the expanded state into the card
- Due that topmost task is always collapsed do not show expanded icon at all
- Minor cleanup

PS:
- set mFirstTask to null if it is not used

Change-Id: Ifaabc035b6b9bd2d4527d5b34a1bca9c80a74e32