Freeze bt_clock_class_priority_map in related notifications

Freeze bt_clock_class_priority_map in related notifications

We don't want the relative priorities of a notification's clock classes
(event, inactivity) to change once it's emitted.

Should a new clock class be needed (with its own priority), you should
create a new clock class priority map.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>