Tangawiki is ginger in Kiswahili.
It's actually 'kwaheri' not kwahari etc.
Stoney tangawizi is essentially what's known as ginger beer in the big wide world. Look for Jamaican ginger beer and it's about the same thing.
I think Coca Cola realised that you couldn't sell soda called 'beer' in Kenya and came up with some creative name to let folks know what indeed they were selling was just ginger soda.