Floss: Don't emit pairing metrics on cancelling

In BlueZ, we don't emit any pairing event in cancelling. This makes
Floss align with BlueZ to better compare their performance.

Bug: 295826875
Test: mma -j32
Test: start pairing attempt to a keyboard and cancel the bond. Verify
there is no pairing event on cancelling by checking the syslog.
Tag: #floss

Change-Id: I1058fc8386ddab6b8acc99c9a3cd3a8ecb4efd0a