Using the keyguard fading away duration for the scrim

Previously we were using a fixed duration and as a result the
scrims duration was much shorter right after boot when the
fading away duration takes 2 seconds. We're now always using
the fading away duration which simplifies also the bypass
transition.

Fixes: 138305791
Test: atest SystemUITests
Change-Id: I2eba68c75c9777ab1ba6ffadadafed3836eb74e3