Fix UserHistoryDictIOUtilsTests by adding @UsedForTesting.

Change-Id: I1bfeeafa002496bb98a385b87437e7f4eb0f6e1d
diff --git a/java/src/com/android/inputmethod/latin/utils/UserHistoryDictIOUtils.java b/java/src/com/android/inputmethod/latin/utils/UserHistoryDictIOUtils.java
index 635afe7..7b81e8b 100644
--- a/java/src/com/android/inputmethod/latin/utils/UserHistoryDictIOUtils.java
+++ b/java/src/com/android/inputmethod/latin/utils/UserHistoryDictIOUtils.java
@@ -70,6 +70,7 @@
     /**
      * Writes dictionary to file.
      */
+    @UsedForTesting
     public static void writeDictionary(final DictEncoder dictEncoder,
             final BigramDictionaryInterface dict, final UserHistoryDictionaryBigramList bigrams,
             final FormatOptions formatOptions) {