Bluetooth: Host: Fix issues with host IRK handling

Fix multiple issues related to the way the host handles
Identity Information related to privacy

1. If the controller provided a public address the IRK
   for this identity would be randomly generated but not
   stored persistenly.

2. Fix the handling of the above issue which was fixed
   for the random address but would initiate settings save
   ID on every boot.

3. Fix the host not using the Vendor Specific HCI commands
   related to retrieving the Identity Root (IR) from the
   controller and using the key diversified function d1
   to generate an IRK as specified in the BT Core spec.

Make sure that a Host generated ID is only saved when it is first
generated.

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