[flexiglass] SYSUI_STATE_NOTIFICATION_PANEL_EXPANDED when occluded

As it turns out, SYSUI_STATE_NOTIFICATION_PANEL_EXPANDED needs to be
false when system UI is occluded.

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

By having it be false, the back navigation works when the camera app is
occluding system UI.

Bug: 349438272
Test: added unit tests expanded
Test: manually verified that back navigation worked while the camera app
was occluding system UI.
Flag: com.android.systemui.scene_container

Change-Id: I3606a217c05365636258690e253096ec98cf9983