Bluetooth: mgmt: Add idle_timeout to configurable system parameters

Bluetooth: mgmt: Add idle_timeout to configurable system parameters

While the configurable system parameters allow controlling the SNIFF
mode parameters, they do not include the idle_timeout parameter
responsible for enabling SNIFF mode.

Add the idle_timeout parameter to allow controlling the idle timeout
of BR/EDR connections.

Signed-off-by: Stefan Sørensen <ssorensen@roku.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>