[flexiglass] Prevent unintentional switching to the Gone scene.

For security purposes, require a separate method in SceneInteractor be
used if wanting to switch to the Gone scene.

Fix: 319298118
Test: manually verified that flexiglass still works and I can unlock,
relock, and unlock the device again
Test: new unit test methods added, existing ones all pass
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: If52184b3a9d26d77c4f2af7e27d5169240601904