lib: msg. iter. inactivity message has a simple CS, not a default CS

The "default clock snapshot" properties of some types of messages come
from the fact that a stream class has a default clock class, and
therefore its streams have a default clock.

A message iterator inactivity message is not related to any stream, so
it doesn't have a "default" clock class: it has a simple clock class,
and therefore a simple clock snapshot.

Update the C and Python APIs to show this.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I0142c4f91217791e3157d37a32f4e2f234afa8d2
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2801
Tested-by: jenkins <jenkins@lttng.org>