Add IPv6 back to kernel config

Not having IPv6 in the kernel causes an assortment of problems for the
VM. Worst of all, the implementation of java.net.ServerSocket evidently
binds to the v6 wildcard address now, making any call to it fail with
some very unintuitive error messages. This should also clear up some of
the logcat complaints from other system services regarding firewall rules
and other networking things.