The Tor Project is currently working on a Tor-enabled smartphone code named Mission Improbable that is hardened for privacy and security. The operating system is CopperheadOS, which is a security hardened fork of the Android Open Source Project. I'd say working with The Tor Project and Copperhead Security and forking the project makes more sense than trying to reinvent the wheel by building a similar secure open source privacy smartphone from the ground up.
Links to official sites:
Tor Phone announcement post
Related ArsTechnica articles:
Tor phone is antidote to Google “hostility” over Android, says developer
Copperhead OS: The startup that wants to solve Android’s woeful security