Implement transition for docking task in recents #1

- When the docking transition is happening, defer updating
the bounds of the home stack until the transition is done.
This is to preserve the scrim which is drawn in the recents
activity.
- Use the PROLONG_AT_START infrastructure to hide the task
in recents when starting the app transition.
- When recents finally get resized at the end of the transition,
reset it's draw state so we don't move the old surface around,
and the new surface gets drawn at the new position, to avoid
flickering.
- Remove hack around not layouting docked divider if it's not
visible, it's not needed anymore and resulted in a wrong
initial position.
- Fix animation selection for docked stack divider.
- Make sure win.moved() always gets called.

Bug: 27607141
Change-Id: I76c35f09461f044a90e2c88335008284b5839cc0