location: Translate accuracy level for geoclue

The enums values of geoclue have gaps in them (so more levels could be
added in future) but enum values of settings don't have such gaps so we
gotta translate between them.

Since desrt says that enums as integers in gsettings are bad, we now
treat accuracy level settings as string.

This fixes the recent regression of geoclue only allowing geiop level
accuracy to apps.

https://bugzilla.gnome.org/show_bug.cgi?id=736479