Wrap NotificationShadeWindowView in a new window root view

The view will only be present when the scene_container flag is
enabled. When the flag is disabled, the view heirarchy will be
unchanged.

Fixes: 283835201
Test: used the Flipper app to verify the flag both on and off
Change-Id: I09362df88a9ec284654f58dc1d061a143c99c865