Fix screen rounded corner flicker.

We've previously always played transitions with a fixed start corner
radius. This should however be an interpolated value based on gesture
progress.

Bug: 230115505
Test: Turn on predictive animations. Try completing a back swipe as well
as canceling a back swipe. Observe corner radius to make sure there's no
flicker.

Change-Id: Iae2517d969ed1b779bced866407b939b3a011b68