Large screens: fully fade in behind scrim before anything else on shade

Before, elements of the shade were starting to fade in, before the
behind scrim was fully faded in.
This made it possible to see the launcher and notifications at the same
time for example.

The fix is to fully fade in the behind earlier.

Test: ScrimControllerTest.java
Test: Manually on-device. See videos.
Fixes: 247119667
Fixes: 265402313
Change-Id: I54475061d530d076a0b2f11a7e23e689e3cccb5e