fingerprint: refactor implementation of custom cancel-message

  - Quite important, add comments

  - Fix possible memory leaks on multiple cancel()-calls, memory for messages
    has been allocated, but never free-ed due to possible problems if
    the notifciation is being forwarded asynchronously

  - Fix error-data whenn bauth already failed to send cancel-command to hardware:
    If ss_fingerprint_cancel() returns with an error, we shouldn't send a notification
    of a successful to Android either

Change-Id: I79dd6369f878e41a4f4058b4c19ff7cff079ebcd