Always suppress scrim for taskbar

Taskbar never needs a semi-transparent scrim drawn by the decor view at
the insets area, because taskbar has the background color on its own if
needed.

Bug: 197615177
Test: SystemUIGoogleBiometricsScreenshotTests
Change-Id: Ic49c8af3ecbefcc83d1bd987812625c7e2f37157