[FIX] point_of_sale: fix numpad behavior on payment screen

Steps to reproduce:
===================
- Add a product to the orderline from the product screen
- Click payment
- Select any payment method
- Enter the amount manually
- Press `+20`, then press `2`, OR press `+20`, then press `6`

Issue:
======
- In the case of `+20` and `2`, the expected result is `202`, but we get `20`
- In the case of `+20` and `6`, the expected result is `206`, but we get `20.01`

Cause:
======
- `formatCurrency` is used for the buffer, but the buffer should not depend on
  the currency

Fix:
====
- Remove the usage of `formatCurrency` and treat the buffer as a string

closes odoo#213294

Task: 4850543
Signed-off-by: David Monnom (moda) <[email protected]>

Author: vpan-odoo

addons/point_of_sale/static/src/app/utils/number_buffer_service.js

@@ -289,11 +289,7 @@ class NumberBuffer extends EventBus {
             // when input is like '+10', '+50', etc
             const inputValue = oParseFloat(input.slice(1));
             const currentBufferValue = this.state.buffer ? oParseFloat(this.state.buffer) : 0;
-            // FIXME POSREF: the `buffer` shouldn't be dependent on the currency.
-            this.state.buffer = this.component.env.utils.formatCurrency(
-                inputValue + currentBufferValue,
-                false
-            );
+            this.state.buffer = (inputValue + currentBufferValue).toString()
         } else if (!isNaN(parseInt(input, 10))) {
             if (this.state.toStartOver) {
                 // when we want to erase the current buffer for a new value

addons/point_of_sale/static/tests/tours/PaymentScreen.tour.js

@@ -46,12 +46,17 @@ registry.category("web_tour.tours").add("PaymentScreenTour", {
             PaymentScreen.remainingIs("42.8"),
             PaymentScreen.changeIs("0.0"),
             PaymentScreen.validateButtonIsHighlighted(false),
+            PaymentScreen.pressNumpad("5"),
+            PaymentScreen.fillPaymentLineAmountMobile("Cash", "105"),
+            PaymentScreen.remainingIs("0.00"),
+            PaymentScreen.changeIs("52.2"),
+            PaymentScreen.validateButtonIsHighlighted(true),
             PaymentScreen.pressNumpad("+50"),
-            PaymentScreen.fillPaymentLineAmountMobile("Cash", "60"),
+            PaymentScreen.fillPaymentLineAmountMobile("Cash", "155"),
             PaymentScreen.remainingIs("0.0"),
-            PaymentScreen.changeIs("7.2"),
+            PaymentScreen.changeIs("102.2"),
             PaymentScreen.validateButtonIsHighlighted(true),
-            PaymentScreen.clickPaymentlineDelButton("Cash", "60.0"),
+            PaymentScreen.clickPaymentlineDelButton("Cash", "155.0"),
 
             // Multiple paymentlines
             PaymentScreen.clickPaymentMethod("Cash"),