[IMP] pos: use iot scale

Before this commit the continuous reading of the scale
was started directly in `show`.

In order for us to be able to use the scale with IoT drivers
this piece of code needs to be overriden, so it was moved to a separate function.

Task: 1892412

addons/point_of_sale/static/src/js/screens.js

@@ -289,7 +289,6 @@ var ScaleScreenWidget = ScreenWidget.extend({
     show: function(){
         this._super();
         var self = this;
-        var queue = this.pos.proxy_queue;
 
         this.set_weight(0);
         this.renderElement();
@@ -314,7 +313,12 @@ var ScaleScreenWidget = ScreenWidget.extend({
             // add product *after* switching screen to scroll properly
             self.order_product();
         });
+        this._read_scale();
+    },
 
+    _read_scale: function() {
+        var self = this;
+        var queue = this.pos.proxy_queue;
         queue.schedule(function(){
             return self.pos.proxy.scale_read().then(function(weight){
                 self.set_weight(weight.weight);