Bluetooth: Add support for background LE scanning

If we have one or more devices with HCI_AUTO_CONN_REPORT we should do
background scanning and emit mgmt_device_found events. This patch
modifies the hci_update_background_scan() function to extend the
conditions needed to trigger scanning, and adds the necessary code to
process_adv_report() to emit mgmt_device_found events.

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