Slim recents: Fix blinking cards on notifyDataSetChanged()

Cards would animate semi-transparent and back after removing
other cards, but sometimes also when actually nothing changed
(but notifyDataSetChanged() called).

Add a custom ItemAnimator to still have the transparency thing
going when expanding/collapsing cards.

Change-Id: I14bcc9408db1b71f47a436cd46ea5a8a4a52ce2d