[bc25] Do not make the scrim opaque when Dual Shade is open.

Flag: com.android.systemui.dual_shade
Fix: 344854041
Test: Manually by opening and closing the dual shade, and doing the same
in legacy mode to ensure existing behavior didn't break.
Test: Unit tests pass.

Change-Id: I7b71bf4e805f2f1426a28c71009e181439966f2e