Added shade bottom corner radius transition for predictive back

The statically set notification scrim bottom corner radius that
was added for predictive back was larger than the screen corner
radius, causing elements behind the shade to be visible. This adds
a smooth transition for the bottom corner radius during predictive back
and fixes the visual bug.

Bug: 273994446
Test: Verified that shade CUJs appear correct on both small and large screens
Change-Id: Ibef677179e2d1ed0bf61d8b09ab06d2459cb6d8f