Change behind-scrim's tint in BOUNCER state to match GM3 surface color.

This is in lieu of relying on the KeyguardSecurityContainer to paint the
background with surfaceColor. It fixes an issue during
KeyguardSecurityContainer's vertical translation, triggered either via a
gesture or during the bouncer dismiss animation, where the wrong color
tone is surfaced and the alpha fades conflict.

Bug: 281965452
Test: unit tests
Test: manually tested bouncer on large screen, incl theme change http://shortn/_DVOtSJtrwy
Change-Id: I67149c6faa2766be6d2537f2315dd2734bdd0447