[flexiglass] surfaceBehindVisibility

Provides a Flexiglass-aware implementation of surfaceBehindVisibility.

Test: manually verified that the launcher is shown and is interactive at
the right times (when the device is unlocked).
Test: unit tests added.
Flag: com.android.systemui.scene_container
Bug: 343297387

Change-Id: I360c14e66be7f247f84d8d7f989be7c9ca9f6b34