am 421c34c1: SipPhone: revise hangup() in SipCall and SipConnection.

Merge commit '421c34c162098efe870574844a7ee49812bbb929' into gingerbread-plus-aosp

* commit '421c34c162098efe870574844a7ee49812bbb929':
  SipPhone: revise hangup() in SipCall and SipConnection.