[FIX] web: FormView: add o_xxs_form_view className on small screens

The form view is styled differently on small screens, and those rules
are based on the presence of the "o_xxs_form_view" className. This
commit correctly toggles it.

Part-of: odoo/odoo#78221

addons/web/static/src/views/form/form_controller.js

@@ -410,6 +410,13 @@ export class FormController extends Component {
             translateAlertData: toRaw(this.translateAlertData),
         };
     }
+
+    get className() {
+        return {
+            [this.props.className]: true,
+            o_xxs_form_view: this.env.isSmall,
+        };
+    }
 }
 
 FormController.template = `web.FormView`;

addons/web/static/src/views/form/form_controller.xml

@@ -2,7 +2,7 @@
 <templates xml:space="preserve">
 
     <t t-name="web.FormView" owl="1">
-        <div t-att-class="props.className" t-ref="root">
+        <div t-att-class="className" t-ref="root">
             <Layout className="model.useSampleModel ? 'o_view_sample_data' : ''">
                 <t t-if="showingTranslateAlert[model.root.resId]" t-call="web.TranslateAlert" />
                 <t t-set-slot="layout-buttons">