Refactoring SIP classes to get ready for API review.

+ replace SipAudioCall and its Listener interfaces with real implementations,
  + remove SipAudioCallImpl.java, most of it is has become part of SipAudioCall,
+ add SipSession and its Listener classes to wrap ISipSession and ISipSessionListener,
+ move SipSessionState to SipSession.State,
+ make SipManager keep context and remove the context argument from many methods of its,
+ rename SipManager.getInstance() to newInstance(),
+ rename constant names for action strings and extra keys to follow conventions,
+ set thread names for debugging purpose.

Change-Id: Ie1790dc0e8f49c06c7fc80d33fec0f673a9c3044