Bluetooth: Add public mgmt function to send New Settings event

A function is needed so that the HCI event processing can ask the mgmt
code to emit a new settings event. This is necessary e.g. when the event
processing does updates to mgmt related states without any dependency of
actual mgmt commands.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>