WeatherClient: set enabled providers after setting a valid location

WeatherClient: set enabled providers after setting a valid location

So far, the GWeatherInfo was given the enabled weather providers
as a parameter, at construction time. Because of the way in
which libgweather was designed, setting the providers right from
the beginning enabled libgweather to use them internally in order
to update its state. Updating the internal state is only relevant
when there is a valid location set, which is not guaranteed at the
time when the GWeatherInfo object is constructed.

In order to fix this, enable no providers at construction time and
only set valid providers after setting a valid location.

https://bugzilla.gnome.org/show_bug.cgi?id=780404