getPreferredNetworkType should return int[] instead of -1 on error.

Bug: 25224221
Change-Id: I6cd3492e93ab1a6cc17fb9385a970e6ad22aa0f4
diff --git a/src/com/android/phone/PhoneInterfaceManager.java b/src/com/android/phone/PhoneInterfaceManager.java
index d61ef59..21b3b22 100644
--- a/src/com/android/phone/PhoneInterfaceManager.java
+++ b/src/com/android/phone/PhoneInterfaceManager.java
@@ -604,7 +604,7 @@
                     if (ar.exception == null && ar.result != null) {
                         request.result = ar.result;     // Integer
                     } else {
-                        request.result = -1;
+                        request.result = null;
                         if (ar.result == null) {
                             loge("getPreferredNetworkType: Empty response");
                         } else if (ar.exception instanceof CommandException) {