Fix toString() of resource Configuration for Mobile network code

The code was compared to 0 when printing. However, 0 can be a valid
value and MNC_ZERO should be used instead for the comparison.

Test: N/A
Change-Id: I9440db1ae779549650518d80da439e0060a0beca

core/java/android/content/res/Configuration.java

@@ -1107,7 +1107,7 @@ public final class Configuration implements Parcelable, Comparable<Configuration
         } else {
             sb.append("?mcc");
         }
-        if (mnc != 0) {
+        if (mnc != MNC_ZERO) {
             sb.append(mnc);
             sb.append("mnc");
         } else {