floss: Send event on connection failure

This can be used to tell the upper layers that connection fails,
therefore they can provide better feedback to the user. Additionally
floss will also use it for metrics.

Bug: 240782154
Tag: #floss
Test: Verify btadapter print warn "Connection to <address> failed"
      when connection failed.
Change-Id: Id2ec0d71bf5392fdc8314349296d71a6af1e76ba