I originally wrote clatd for the Nokia n900 phone back in 2010. After porting it to Android in 2012, Google added it to Android 4.3 in 2013. Google now maintains it as part of the Android OS.
clatd performs the ipv4->ipv6 translation as part of 464xlat (RFC6877). It works in conjunction with the plat server run by the service provider.
The source code for clatd is part of android git.
Related work is RFC7278: Sharing /64 3GPP Mobile Interface Subnet to a LAN, which describes how to share IPv6 with a tethered network. This was added somewhere around Android 6.0