Bluetooth: host: Allow requesting new security when security changed

Allow to request a higher security level during the key distribution
phase.

This is required by ATT and L2CAP since they only react to the encrypt
change event where they resend the current request.
The current request might require a higher security level still and
might have to request a higher security level before the pairing
procedure has been finished.

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