am 960b31e3: Merge "SipService: unset session listener if listener is gone." into gingerbread

Merge commit '960b31e3b3b40f92665be4b7a79da7f05ea19751' into gingerbread-plus-aosp

* commit '960b31e3b3b40f92665be4b7a79da7f05ea19751':
  SipService: unset session listener if listener is gone.