Slim Recents: Always show topmost task collapsed

PS2: Also restore expanded state of topmost task when on homescreen
PS3: Fix code style

Change-Id: I64956410fb389d8e880979a84b51321d8b372217
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