[flexiglass] Don't render lockscreen when occluded.

If an occluding activity is visible and the lockscreen scene content is
being rendered (for example, for a transition from lockscreen to shade),
rener the lockscreen as an empty box.

Fix: 350930909
Test: added unit tests
Test: manually verified that the lockscreen elements don't render over
the occluding camera activity, even when expanding/collapsing the shade
Test: manually went through all the other scenes, when locked and
unlocked
Flag: com.android.systemui.scene_container

Change-Id: I600c77e574820c4a18a7e447826719f98a75e052