hci, l2cap: add the handler for Encryption Key Refresh Complete Event

This sets the handler in hci, adds the l2cap API to handle the key
refresh logic and adds the notification to parties who care about this
event.

BUG=chromium:805112
TEST=build

Change-Id: I0c2a67a5bedfdeddd45005c84b73278f579609d3
3 files changed