[flexi] Test only calls onDisplayBlanked when SceneContainer disabled

onDisplayBlanked doesn't get called when the SceneContainer
flag is enabled. Add a SceneContainer.isEnabled check
in the test to better reflect what actually happens.

Flag: com.android.systemui.scene_container
Test: atest DozeScrimControllerTest
Fixes: 416709457
Bug: 354910543
Change-Id: I18b92802d981b366332962f7a1faace456c383e3