Allow notification shelf to be toggled off.

Android auto's notifications UX does not have a notification shelf. Add
a config that hides the shelf.

Ideally, the shelf should be conditionally created, but it is closely
coupled through system UI. This change would require a larger refactor,
so just allow it to be hidden for now.

Also, update some areas that looked at the height of the notification
shelf. If it has a visibility of GONE, then don't grab the height.

Test: booted up phone and Android Auto headunit to verify
Bug: 33210494
Change-Id: I66b1eb956838e26bf695da01dab9ba5349ee321e