Disable snap targets that make apps <220dp

- Make minimal task size 220dp.
- Disable upper and lower targets if they result in less
than 220dp task size.
- If even the middle target doesn't allow 220dp task size,
disable entering split screen altogether.

Bug: 26451260
Change-Id: I06e358c9b3da0172c5def75cdadf975f87f9fa57