Call onLauncherTransitionPrepare() and End() from pinch.

This makes the pinch transition more consistent with other transitions.
One immediate benefit of this is that it updates adjacent overview
panels during pinch, regardless of whether they are completely visible.

Previously the adjacent panels' alphas weren't always reset to 0.
Specifically, if you made a small pinch from workspace, which
canceled and went back to workspace, adjacent pages retained a
slightly visible panel.

Bug: 27676309
Change-Id: I7e79fddec31cd649e0811e4524b9a9a501c627f9