Bluetooth: Settings: Fix generated identity not persistently stored.

Fix an issue where the generated identity was not permanently stored.
This resulted in being unable to reconnect after bonding when using
privacy, since a new local IRK was generated on reboot.

When settings is enabled the application is responsible for loading
identities and possible creating its own identities.
When settings_load is called and no identities has been created or found
in persistent storage a new identity will be created.
Since bt init has not been finalized bt_id_create will not make a call
to bt_settings_save_id. So we need to make sure that this identity will
be stored.

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