Remove bottom corner radius from notification scrim

Removes the bottom corner radius from the notifications scrim
except in split shade (or when predictive back animation is running),
to avoid instances of mismatch between the rendered corner and
the actual device corner.

Bug: 278957007
Test: verified scrim looks correct in accordion and split shade
Change-Id: Ia3e575c2a306bdb73adad339e27692dea840d093