Federal Communication Commision regulates cell phones. Federal law preempts state law. Any California law could be nulified by the FCC.
Federal Law only pre-empts state law when there is a contradiction. Is there a federal law that specifically says cell phones must not have a remote killswitch?
The endgame of this legislation for totalitarian-loving legislators is that ultimately that's what will happen - they propose onerous conditions on the manufacturers and carriers who can't say no to the 12th largest market  in the world (probably even more important for cell phones), who then complain and ask the Feds to step in and propose their own laws that supersede the CA law, which then requires their client states and treaty members to follow along (for those nice economic treaties mandating matching legislation to promote copyright and favorable business environment for US corporations).
At each point in this chain of corruption, the law can be "enhanced" to provide more power to the TSA, CIA, FSB, Mossad, etc so wealth interests can keep increasing their profit margins.
Sounds like a win/win/win to me*
* Only wealthy and powerful may participate. Political "donations" apply. Offer only valid in participating jurisdictions.