Don't start task from quickswitch until scrolling finishes

Before, we did this by extending the window/launcher animation to
match the scroller duration. But now that we are using springs to
control the scroller, that duration is not really accurate. So
instead, we now let the window/launcher move at its own pace, and
wait for both that animation and the scroller to finish before
calling onSettledOnEndTarget().

Bug: 147302669
Change-Id: I37a9dd3eea17ebe663c33c3a4478b1b53a63dcc2