Gadgetbridge: Add Google Fit support.

[RustyRaptor]

It would be nice to add support for google fit. I understand this app is meant to avoid sending data to anyone but some people use it as an alternative simply because the official app doesn't support anything older than android 4.4. Maybe just make it optional.

[relan]

Free software should _not_ be tainted by proprietary software support. I hope Gadgetbridge will never do this.

[IzzySoft]

This is already dealt with in #248. And to quote one of our devs:

We will not add this directly to Gadgetbridge, that's for sure. We could support this scenario through other means, though, e.g. through export and an additional sync tool.

[danielegobbetti]

Duplicate of #248 .
Further, as correctly already reported, this is a WONTFIX for the main app.