Multi-shade foundation - integration (5/5).

Early foundation for the multi-shade framework.

Integrates the basic framework into the current
NotificationShadeWindowView. Does not do any extra work to make it work.

The change is behind the DUAL_SHADE flag which is currently off.

https://drive.google.com/file/d/1nmbIQuwvWZ0VqnWHxqfcpxbZqapbHtmZ/view?usp=sharing

Bug: 272130181
Test: Manually tested end-to-end. When the flag is off, there is no
impact on system UI behaviour. When the flag is on, dual shade appears
on top of the normal shade. It even accepts input and the shades can be
expanded and collapsed.
Flag: DUAL_SHADE

Change-Id: I0fc00d05a1fc9451ca92a53638dc3cbc7a058459