[flexiglass] Fixes "wallpaper not visible on AOD" bug.

The reason the wallpaper wasn't visible was because the notification
shade window was being set to opaque which tells the window manager to
not render windows that are below it on the z-axis.

The reason it was being set to opaque was because ScrimController was
setting the alpha of one of the scrims to 1.0 when Flexiglass was on but
to 0.2 when Flexiglass was off.

The reason that was happening was because the alpha of the scrim depends
on the "panel expansion" amount (from 0.0 to 1.0) and this is a concept
that no longer exists in Flexiglass so the legacy code was treating this
as a 1.0 at all times.

The CL adds the panel expansion concept but clearly marks it as
@Deprecated and legacy and ties the amount to Flexiglass state. Finally,
it connects the logic in ShadeTransitionController to the old-new
concept when Flexiglass is on and disables the old way.

Fix: 323072803
Test: unit test added for PanelExpansionInteractor
Test: manually verified that the wallpaper is visible in AOD
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: Iea0cb312aab442ac18fb39b17376fc2b22090d65