Keeping track of which cell tower your phone is closest to is fundamental to cellular technology. You can't make or receive calls unless Verizon, T-Mobile, or whichever carrier you have knows which cell to communicate with you through. Regardless of whether Google is tracking this, your carrier certainly is, and with a warrant (or a national security letter), law enforcement can definitely access this data. If you're worried about hackers, I can guarantee you Google protects this data more securely than your carrier does.