Fixing ModelPreload cancelling existing load

When a model preload call was made while the loader task is running
(eg: on enabling/disabling icon theme, Launcher reloads and then
launcher preview start a model-preload), it would cancel the original
loader and then start a new loader with empty callbacks. So the
model indeed get loaded, but the original callbacks never got notified
of it.

> Instead we only start preload if an existing task is not running.
> Also when preloading, we use existing callbacks, instead of using
  empty callbacks

Bug: 193851085
Bug: 195155924
Test: Verified repro steps
Change-Id: I0a96310be8489756f364aa2a12e4345e1418733d