Bluetooth: host: Refactor out identity handling from hci_core to id

Refactor out the identity handling from hci_core to its own source file
in id.c
Identity consistes of managing the identities of the device, the privacy
feature which hides the identities. And handling of the identity
resolving list in the controller, needed to support privacy-enabled
remote devices.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>