location: Update available accuracy on max accuracy changing

We translate 'On' to available accuracy level but if available accuracy
level later changes, we don't update available accuracy level accordingly
and hence limit the accuracy of apps.

E.g if available accuracy level is 'city' and geolocation is enabled,
the max accuracy level would be 'city' and apps can't get higher than
that. Now if user plugs in GPS, the available accuracy level will change
to 'exact' but without this patch max accuracy level will remain to be
'city' and apps will not be able to use the GPS.

https://bugzilla.gnome.org/show_bug.cgi?id=731882