[Dual Shade] Dynamically disable transparency when blur is disabled.

The blur applied to WindowRootView can be conditionally disabled at
runtime by a system property, or by the battery saver mode. The shade
container should therefore only apply a transparent background if blur
is enabled.

Bug: 370556579
Test: Added unit tests.
Flag: com.android.systemui.scene_container
Change-Id: Ie74b8c90547f0f664e5d3c020059ac507f85f36e