Bluetooth: Add support for encryption key refresh

With LE/SMP the completion of a security level elavation from medium to
high is indicated by a HCI Encryption Key Refresh Complete event. The
necessary behavior upon receiving this event is a mix of what's done for
auth_complete and encryption_change, which is also where most of the
event handling code has been copied from.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>