SipPhone: revise hangup() in SipCall and SipConnection.

Make them DISCONNECTED immediately. Don't enter DISCONNECTING state and wait
until SipSession ends the session. SipSession will get timed out eventually
but PhoneApp/user don't need to know this detail and wait.

This should fix the bug:
http://b/issue?id=3027719

Change-Id: Ida5a1bd09d08b9d591721384b4978127619aab51