Improve Split portrait gesture animation

* Crop out taskbar from bottom thumbnail for vertical split
* TODO: Need to re-calculate thumbnail sizes if taskbar
 is stashed. There's also a very slight rounding error
 somewhere even in the unstashed case that needs to be
 revisited

Bug: 219411750
Test: Start gesture animation in split in potrait
Change-Id: Id6653a352d1eac54b205f3efca740b4e56c7facf