Imsdroid sends 481 in reply to an ACK after a call is cancelled

```
What steps will reproduce the problem?
1. Please see the SIP dialog from the attached trace

What is the expected output? What do you see instead?
After sent the 487 request, imsdroid receives an ACK and replies with a "481 -
Dialog/Transaction Does Not Exist"

What version of the product are you using? On what operating system?
Tested on Imsdroid v2.0.520 on Samsung GS2 running Android 4.0.4

Please provide any additional information below.
Please refer to section 3.8 of http://www.rfc-editor.org/bcp/bcp75.txt

```

Original issue reported on code.google.com by `massimo....@gmail.com` on 5 Dec 2012 at 4:03

Attachments:
- [cancel-call.pcap](https://storage.googleapis.com/google-code-attachments/imsdroid/issue-420/comment-0/cancel-call.pcap)

Start by examining the attached cancel-call.pcap and compare the ACK handling with section 3.8 of the referenced BCP 75. Reproduce the sequence on Imsdroid v2.0.520 with Android 4.0.4 if possible; done means the ACK after the 487 response is handled without the reported 481 response.

Written by the indexing model from the issue text.