[WifiSetup] Tablet landscape layout

Created setup_preference for sw600dp-land, modeled after
template_with_header in Setup Wizard. Copied over wide illustration,
which is again a placeholder and needs real graphics from UX.
Renamed SetupWizardHeader to SetupWizardIllustration. Added
aspectRatio attribute to SetupWizardIllustration that, will set the
aspect ratio of the top padding. If 0 or not specified, padding will
not be set.

Bug: 16349429
Change-Id: Ic2623ee0c0ee8864383d22da006b418b8c371bf0