Improve splitscreen swap animation

Motion quality polish. The swap animation now has a new duration and interpolator, new rounded corners, and animates one of the split tasks "behind" the divider with a shrink-and-grow motion.

Also ensured that another swap cannot be triggered while a swap is already in progress.

Fixes: 291893598
Test: Visually confirmed at 10x slowdown
Flag: EXEMPT bugfix
Change-Id: I039c29c8a1a2c0400b5390a66bddb6d8341481f8