Slim recents: don't cancel preload when toggled

Cancelling slim recents preload after toggling causes unexpected behaviour.
This scenario usually shouldn't happen, but let's safeguard slim recents
against it either way.
(It's happening for me using hw buttons, probably a bug there; but
safeguarding slim recents might be a good idea either way)

Change-Id: Id1f73626a49e7c35346d69fb6a6144e2ebdd4c8f