[bc25] Fix notifications shade rendering over lockscreen in Dual Shade.

* Scale the shade height when the shade expands/collapses.
* Do not show the legacy "notifications scrim".
* Avoid clipping notifications.
* Show HUN.
* Ensure transition tracking in `Notifications.kt` correctly handles
  shade overlays.

Bonus:
* Remove some unused functions and variables.
* Remove knowledge of `ShadeMode` from `Notifications.kt`.

Bug: 356596436
Flag: com.android.systemui.scene_container
Flag: com.android.systemui.dual_shade
Test: Manually tested by expanding and collapsing the dual-shade
notifications shade over both lockscreen and gone.
Test: Existing unit tests still pass.

Change-Id: Ib0a509ecf06201b7d04dcc79764236852a04bfcc