[flexiglass] StatusBarState is SHADE when occluded.

As it turns out, the state needs to be SHADE when system UI is occluded.

To make sure of this, I compared the output of
adb shell dumpsys activity service com.android.systemui/.SystemUIService StatusBarStateControllerImpl | grep mState
when the camera app is occluding system UI in pre-Flexiglass to Flexiglass.

By having it be SHADE, the codepath in
CentralSurfaces.showBouncerOrLockScreenIfKeyguard doesn't actually try
to show the bouncer each time that visibility is changed - which was
continuously causing a change to the bouncer scene behind the occluding
camera activity.

Bug: 349438272
Test: added unit test
Test: manually verified that the scene isn't being changed to bouncer
behind the occluding camera app, this is done using adb logcat -s
SceneFramework
Flag: com.android.systemui.scene_container

Change-Id: Id12cfed12ed31909afc4724e79c2932913c1f6ed