support data from google fit activity

Google Fit data can also come in via the [existing activity](https://www.openhumans.org/activity/google-fit/) as data source (["direct-sharing-193"](https://www.openhumans.org/api/public/projects/?id=193)) – we should support that too. :) (I think @BenjaminHCCarr might have mentioned this issue, maybe during community meeting.)