[FIX] web: x2m not editable inline with save & new

Before this commit, in a form view with an x2many in non-editable list mode,
creating a record by clicking on "Save&New" allows it to be edited inline
despite the list view not being editable. The purpose of this commit is
to prevent this from happening.

How to reproduce:
- Go to a form view with an x2many field in non-editable list mode
- Add a record
- Edit the new record
- Click on Save&New
- Edit the new record

Before this commit:
    The new record is in edition in the list view

After this commit
    The new record is not in edition in the list view

Part-of: odoo/odoo#141269

addons/web/static/src/views/fields/relational_utils.js

@@ -587,6 +587,7 @@ export class X2ManyFieldDialog extends Component {
             if (await this.record.checkValidity({ displayNotification: true })) {
                 await this.props.save(this.record);
                 if (saveAndNew) {
+                    await this.record.switchMode("readonly");
                     this.record = await this.props.addNew();
                 }
             } else {

addons/web/static/tests/views/fields/one2many_field_tests.js

@@ -9846,7 +9846,7 @@ QUnit.module("Fields", (hooks) => {
                 <form>
                     <field name="p">
                         <tree>
-                            <field name="display_name"/>
+                            <field name="display_name" widget="char" class="do_not_remove_widget_char"/>
                         </tree>
                         <form>
                             <field name="display_name"/>