Fix white scrim background being white over hub on dream

Each ScrimState is a separate class instance, so calling prepare in an
attempt to reuse code didn't work as intended.

Also updated tests to properly test the intended behavior instead of
passing trivially. Verified updated openShadeOnHubOverDream test fails
without the corresponding ScrimController change and passes afterwards.

Bug: 323036128
Fixed: 323036128
Test: atest ScrimControllerTest
      also manually verified on device
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: I545dddfd42117310bb5630c2b85a2715469c482b