Tweak and fix bugs with new app close transition.

* Animates LauncherRootView instead of DragLayer to avoids the flashing that
  can occur when the overlay callback also changes the DragLayer alpha.
* To avoid the scrim's hard line: we hide it and then fade it in later.
* Launcher animation was wrong in landscape mode.

Change-Id: I7673228f5ed8bb72d7393f3d0769577b262f286f