lib: Pass raw snapshot value to bt_message_inactivity_create()

Having both
  bt_message_inactivity_create(bt_self_message_iterator *message_iterator,
      const bt_clock_class *default_clock_class);
and
  bt_message_inactivity_set_default_clock_snapshot(
      bt_message *msg, uint64_t raw_value);
is redundant given that setting the clock snapshot value is required to
send the inactivity message down the graph.

This commit makes it so that the value of the snapshot is required at
the creation of the inactivity message.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>