Prevent dial() return null in a racing condition.

The racing condition happens between dial() returns and
the first GET_CURRENT_CALLS query gets handled.

If GET_CURRENT_CALLS gets handled before dial() finishs, the pendingMO can be set
to null in handlePollCalls() so that dial() will return null. This null connection causes
error in PhoneUtils.placeCall().

The Synchronized dial() and handlePollCalls() Methods will make sure the
dial() returns before the first GET_CURRENT_CALLS gets handled.

bug:6028290
Change-Id: I41b024760acb7dd13b342866180dffe3fdbe1c03

Author: John Wang

telephony/java/com/android/internal/telephony/gsm/GsmCallTracker.java

@@ -167,7 +167,7 @@ public void unregisterForVoiceCallEnded(Handler h) {
     /**
      * clirMode is one of the CLIR_ constants
      */
-    Connection
+    synchronized Connection
     dial (String dialString, int clirMode, UUSInfo uusInfo) throws CallStateException {
         // note that this triggers call state changed notif
         clearDisconnected();
@@ -406,7 +406,7 @@ public void unregisterForVoiceCallEnded(Handler h) {
         }
     }
 
-    protected void
+    protected synchronized void
     handlePollCalls(AsyncResult ar) {
         List polledCalls;