Fixed bug with black background when exiting a wallpaper window.

* Defer hiding the wallpaper until all animations are done. We were
previously hiding the wallpaper once the closing app animation is
done which leads to the background of the opening app animation
going black since there is nothing to show there.

* Don't set the wallpaper_close_enter animation blackground
to black. This will also cause the background of the opening app
animation to go black once the wallpaper closing app animation is
done.

* Don't set task_open_enter animation blackground to black. This
can cause an undesired black frame during transition if the new
task is created on the home stack instead of an application stack.

Bug: 17098816
Bug: 19454410
Change-Id: I0e63d72b1748d7d2424119b3176f48476293af92