Came to the comments to say exactly this.

Open source, with thousands of eyeballs looking at it, tends to be more secure: obscurity is not security.

Android is open source (AOSP). The security issue there is the carriers, and the lag they introduce between security fixes and updated ROMs getting installed on devices (IMO, a major reason to get root access on a phone is to take control of that updating yourself - as well as direct per-app firewall control.)