Chore: Remove public vars in setting package (#81018)

Removes the public variable setting.SecretKey plus some other ones. 
Introduces some new functions for creating setting.Cfg.

pkg/services/anonymous/anonimpl/impl.go

@@ -88,7 +88,7 @@ func (a *AnonDeviceService) tagDeviceUI(ctx context.Context, httpReq *http.Reque
 
 	a.localCache.SetDefault(key, struct{}{})
 
-	if setting.Env == setting.Dev {
+	if a.cfg.Env == setting.Dev {
 		a.log.Debug("Tagging device for UI", "deviceID", device.DeviceID, "device", device, "key", key)
 	}