Bluetooth: SMP: Make public key handling more robust

Update code to handle other users of the public key generation APIs
by fetching the current public key at the beginning of each SMP
session. This is particularly important if someone creates the (rather
odd) combination of Mesh and SMP where Mesh will regenerate a new
key pair after provisioning.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>