Yes, Kirk Cousins is still the clear favorite to start Week 1 for the Raiders. The organization has been explicit about not rushing rookie Fernando Mendoza (the No. 1 overall pick), with HC Klint Kubiak and the front office viewing Cousins as the bridge starter while Mendoza develops in the Shanahan system (from the Kirk Cousins page).
The Raiders' whole plan is built around patience — Cousins is there to mentor Mendoza and keep the offense stable while the rookie absorbs Kubiak's playbook behind the scenes. Everyone from Albert Breer to ESPN's Aaron Schatz has confirmed this as of late July.