[bc25] Show notifications in the `NotificationsShade`.

This isn't done yet; due to b/333116415, the notifications don't render
on large screens. The appearance will also be adjusted in a future CL to
match UX specs.

Test: Manually tested by opening the notifications shade on both
 lockscreen and gone scenes.
Test: Unit tests build and pass.
Bug: 337849926
Bug: 340553529
Flag: com.android.systemui.dual_shade
Change-Id: Ic46319411536b4a7503e18b6c20c09dc40caf331