[IMP] project: do not display private on project if task is subtask

Before this commit, since the display_project_id field has been removed and
the project_id field could be unset on subtasks (default value as the
`display_project_id` field), the widget `project_private_task` consider
a task as private if the task has no project_id set.

This commit changes the widget to check if `is_private` field is true
to display the `Private` on `project_id` field value.

task-3230063

X-original-commit: b0a8a9aedb3b5cfb2acb6ff3b10a77c279bda8fd
Part-of: odoo/odoo#115781