mail-notification: Use different icon name in the notification under Flatpak

The Flatpak renames the 'evolution' icon to the 'org.gnome.Evolution',
thus use that one under Flatpak, to have it available in the notification
bubbles on the host system.

src/plugins/mail-notification/mail-notification.c

@@ -538,7 +538,11 @@ new_notify_status (EMEventTargetFolder *t)
 			additional_messages), additional_messages);
 	}
 
-	icon_name = "evolution";
+	if (e_util_is_running_flatpak ())
+		icon_name = "org.gnome.Evolution";
+	else
+		icon_name = "evolution";
+
 	summary = _("New email in Evolution");
 	escaped_text = g_markup_escape_text (text->str, -1);