Make addAddress locale safe

Using regular string concatenation to avoid unexpected
results in some locales.

Change-Id: I47dd5e174c4a2e88dc18e014002820cdbf63fcad

core/java/android/net/VpnService.java

@@ -329,7 +329,7 @@ public class VpnService extends Service {
                 throw new IllegalArgumentException("Bad address");
             }
 
-            mAddresses.append(String.format(" %s/%d", address.getHostAddress(), prefixLength));
+            mAddresses.append(' ' + address.getHostAddress() + '/' +  prefixLength);
             return this;
         }