Bluetooth: implement read/set default system parameters mgmt

This patch implements the read default system parameters and the set
default system parameters mgmt commands.

Signed-off-by: Alain Michaud <alainm@chromium.org>
Reviewed-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>