[flexiglass] Fade out the lock screen UI when opening an overlay.

Additionally, rename some of the transitions to clarify whether they
refer to the corresponding shade scene or shade overlay.

Fix: 433704325
Fix: 438497154
Test: Manually tested by opening both shades over lock screen, as well
 as the bouncer.
Flag: com.android.systemui.scene_container
Change-Id: I123a9d7578c88c8dcff02a14bff6702cee9c46a4