PeerConnection(java): Add OnRenegotiationNeeded support

Also:
- Make PeerConnectionObserver::OnRenegotiationNeeded() pure virtual to avoid
  this sort of mistake in the future.
- Sprinkle @Override annotations on some callback definitions that were missing
  them.
- Fix a JNI method-signature-lookup typo (s/(V)V/()V/) in PCOJava::OnError()
- Add an explicit ScopedLocalFrameRef to PCOJava::OnError() to match all other
  C++-fired callbacks, for consistency.

BUG=2771
R=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/6829004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5376 4adac7df-926f-26a2-2b94-8c16560cd09d

talk/examples/android/src/org/appspot/apprtc/AppRTCDemoActivity.java

@@ -419,6 +419,11 @@ public class AppRTCDemoActivity extends Activity
           }
         });
     }
+
+    @Override public void onRenegotiationNeeded() {
+      // No need to do anything; AppRTC follows a pre-agreed-upon
+      // signaling/negotiation protocol.
+    }
   }
 
   // Implementation detail: handle offer creation/signaling and answer setting,