Reset leashes before starting taskbar app pair launch animation

* This can be launched from desktop windowing, which means the
surfaces of the apps can start in any position/crop.
* Default the leash tasks to start at 0, 0 and let their parent
surfaces determine correct starting position
* Set the stage roots to start at the correct end bounds

Test: Launched from workspace, DW, taskbar
Fixes: 372837890
Flag: EXEMPT bugfix
Change-Id: Ie2cd012e117f645794e5119485cc50d8e1dd9cdf