[FIX] mail: fix the position of the emoji widget in mobile

Bug
===
Since 569d35bc a fix have been provided
to fix the vertical position of the emoji widget in mobile view.

But the fix have been done in a wrong way and we should use the Odoo CSS
class instead to detect mobile (and not a bootstrap media query).

Task 2253851

closes odoo/odoo#65070

X-original-commit: 72f7acde
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mail/static/src/scss/emojis.scss

@@ -54,17 +54,13 @@ $o-mail-emoji-height: 2rem;
     bottom: 40px;
 }
 
-@include media-breakpoint-down(sm) {
-    // This will fix the position issue for emoji widget on mobile screen.
+
+.o_xxs_form_view {
     .o_mail_emojis_dropdown {
         bottom: 50px;
     }
-    .o_mail_emojis_dropdown_textarea {
-        bottom: 40px;
-    }
     .o_mail_add_emoji {
         .dropdown-menu {
-            // This will fix the overflow issue for emoji dropdown on mobile screen by setting a max-width.
             max-width: 320px;
         }
     }