[Dual Shade] Settings page for switching between Dual & Single shades.

It is available under:
 Settings > Notifications > Notifications & Quick Settings.

Bug: 410541004
Bug: 409228328
Test: Added unit tests.
Test: Tested manually by opening the setting in the Settings app,
 changing the setting, and observing that the dual shade was correctly
 enabled and disabled.
Flag: com.android.systemui.scene_container
Change-Id: If8273885ac43ce527f86812ca055f0b86981cbc6