SipService: reduce the usage of javax.sdp.*.

After this change, SipAudioCallImpl is the only place still using it.

Change-Id: I5693bffa54f9e19cbfa70b45dfcf40fba04dedbb
diff --git a/services/java/com/android/server/sip/SipService.java b/services/java/com/android/server/sip/SipService.java
index 626b488..563ce58 100644
--- a/services/java/com/android/server/sip/SipService.java
+++ b/services/java/com/android/server/sip/SipService.java
@@ -53,6 +53,9 @@
 import java.util.TreeSet;
 import javax.sip.SipException;
 
+/**
+ * @hide
+ */
 public final class SipService extends ISipService.Stub {
     private static final String TAG = "SipService";
     private static final int EXPIRY_TIME = 3600;
@@ -442,7 +445,7 @@
 
         @Override
         public void onRinging(ISipSession session, SipProfile caller,
-                byte[] sessionDescription) {
+                String sessionDescription) {
             synchronized (SipService.this) {
                 try {
                     if (!isRegistered()) {