Actually call LauncherTransitionable.onLauncherTransitionStep().

Previously, it was only called at the start and end of the transition;
now it is called as the animation interpolates. Specifically, a dummy
ValueAnimator is played alongside the transition animation and calls
dispatchOnLauncherTransitionStep() as it goes.

One place where this is important is in Workspace, where
mTransitionProgress is used to determine things like whether the
workspace should accept a drop - hence the bug that caused apps dragged
from All Apps to vanish when dropped before the transition ended.

Bug: 24215358
Change-Id: I32cd633c53557305caf84e87c9a4d4f07eef2223