Move tasks to fullscreen stack when dismissing docked stack.

When we dismissing a docked stack we don't want to get rid of it,
but instead make it available in the full screen stack. We need to
make sure that it goes into the bottom of the stack and prevent it
from taking focus or running any animation, so the operation is
invisible to the user.

Bug: 25840497
Bug: 25823213
Change-Id: Ibd08d7b723a2847d98c435f79541d5eb43587064