Enable DISALLOW_DUPLICATE_FIELD_IN_SKELETON since U

It was supposed to be enabled in S, but we missed it due to
https://r.android.com/1490737

Test: android.text.format.DateFormatTest
Change-Id: I0f6495bf2e4434396606743da1e32a39a54a0b19

core/java/android/text/format/DateFormat.java

@@ -170,7 +170,7 @@ public class DateFormat {
      * mean using 12-hour in some locales and, in this case, is duplicated as the 'a' field.
      */
     @ChangeId
-    @EnabledSince(targetSdkVersion = Build.VERSION_CODES.CUR_DEVELOPMENT)
+    @EnabledSince(targetSdkVersion = Build.VERSION_CODES.UPSIDE_DOWN_CAKE)
     static final long DISALLOW_DUPLICATE_FIELD_IN_SKELETON = 170233598L;
 
     /**