[flexiglass] Adds falsing protection for overlays.

Fix: 390686998
Test: unit tests added
Test: manually verified ability to open dual shade on top of lockscreen
and gone scenes
Test: teporarily hard-coded falsing to always reject transitions and
manually verified that I can't switch to dual shade overlays on top of
either lockscreen or gone scenes
Flag: com.android.systemui.scene_container

Change-Id: I3367e1db3305a16ef0e1e966e81289a5c1995120