Define externally accessible contants with 'var' instead of 'const'

Just as we did with classes, define other constants that are (or
may be) used from other modules with 'var' to cut down on warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=785084

js/ui/status/location.js

@@ -22,7 +22,7 @@ const ENABLED = 'enabled';
 const APP_PERMISSIONS_TABLE = 'gnome';
 const APP_PERMISSIONS_ID = 'geolocation';
 
-const GeoclueAccuracyLevel = {
+var GeoclueAccuracyLevel = {
     NONE: 0,
     COUNTRY: 1,
     CITY: 4,