Fix bouncer scrim animation on split shade

The problem was caused by a wrong expansion fraction being applied
to the scrim when on lockscreen.
That fraction was only intended for non-lockscreen.

+ Drive by refactor to make sure the fraction is always between 0 and 1.

Fixes: 236394769
Test: Manually
Test: ScrimShadeTransitionControllerTest.kt
Change-Id: I63de0f7beda616772e7d97498be91b712d477d57