[FIX] web: readonly monetary field onchange currency

Before this fix:
    A readonly monetary field was not updating
    its currency symbol when selecting another currency.

Scenario:
    1. Activate multi-currency with $ (default) and €
    2. In hr_expense, go to My Expenses to Submit
    3. Create a new expense
    4. Select 'EUR' as currency
        => Unit Price is correctly updated with €, but not Total.

With this fix:
    Total displays the correct currency in edit mode.

Explanation:
    Monetary fields in readonly are relying on _formatValue() to
    display their value.
    _formatValue() uses this.formatOptions.currency, which stores
    the currency at loading of the field. We must update its value
    whenever there is a change of currency.

opw-783378

addons/web/static/src/js/fields/basic_fields.js

@@ -638,6 +638,7 @@ var FieldMonetary = InputField.extend({
         var currencyField = this.nodeOptions.currency_field || this.field.currency_field || 'currency_id';
         var currencyID = this.record.data[currencyField] && this.record.data[currencyField].res_id;
         this.currency = session.get_currency(currencyID);
+        this.formatOptions.currency = this.currency; // _formatValue() uses formatOptions
     },
 });