Avoid overlap when fading notifications in

This CL reduces the number of awkward overlaping frames, where the scrim
is partially visible on top of the app while notifications are also
semi-transparent.

One great benefit of this CL, as well, is that it avoids overdraw,
drawing notifications after the shade window becomes opaque.

Test: using jank scripts
Test: manual
Fixes: 189964129
Change-Id: Ie2fe23aba101bc4c4a5692f999d672e79ddf8e0a