SipService: deliver connectivity change to all sessions.

+ add DATA_CONNECTION_LOST to SipErrorCode
+ convert it to Connection.DisconnectCause.LOST_SIGNAL in SipPhone

http://b/issue?id=2992548

Change-Id: Ie8983c1b81077b21f46304cf60b8e61df1ffd241

voip/java/android/net/sip/SipErrorCode.java

@@ -47,5 +47,8 @@ public enum SipErrorCode {
     INVALID_CREDENTIALS,
 
     /** The client is in a transaction and cannot initiate a new one. */
-    IN_PROGRESS;
+    IN_PROGRESS,
+
+    /** When data connection is lost. */
+    DATA_CONNECTION_LOST;
 }