Fix UserHistoryDictIOUtilsTests by adding @UsedForTesting.

Change-Id: I1bfeeafa002496bb98a385b87437e7f4eb0f6e1d

java/src/com/android/inputmethod/latin/utils/UserHistoryDictIOUtils.java

@@ -70,6 +70,7 @@ public final class UserHistoryDictIOUtils {
     /**
      * Writes dictionary to file.
      */
+    @UsedForTesting
     public static void writeDictionary(final DictEncoder dictEncoder,
             final BigramDictionaryInterface dict, final UserHistoryDictionaryBigramList bigrams,
             final FormatOptions formatOptions) {