[flexiglass] Clicks on shade empty space go back to lockscreen.

Instead of attempting to enter the device and/or show the bouncer.

It doesn't quite fix b/330696389 yet because the clicks aren't being
passed to the clickable in the shade scene composable yet.

Bug: 330696389
Test: unit tests ammended
Test: manually verified that clicking on the empty space in the shade
below the notifications still doesn't do anything, so no damage done
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: Id4e6a2abf77d43818a172381751a0aebed9196cf