core: Expose the last bonding attempt timeout on retry

core: Expose the last bonding attempt timeout on retry

One of the interesting values for pincode plugins is the timeout of a
bonding attempt. For keyboards supporting any random pincode as long as
it is also typed in the keyboard, the timeout until it fails is in the
order of the seconds to allow the user type the pincode on the bluetooth
keyboard. In the case of a dumb keyboard accepting only a fixed pincode
the timeout before the keyboard fails is in the order of a fraction of a
second.

This patch computes the elapsed time between the pairing started or the
pin code was sent to the device and the device returns with an error.
This measured duration is exposed in milliseconds to the plugins when
retrying the bonding attempt.