Forget user history

...src/com/android/inputmethod/latin/ContactsDictionary.java

...c/com/android/inputmethod/latin/ExpandableDictionary.java

java/src/com/android/inputmethod/latin/LatinIME.java

java/src/com/android/inputmethod/latin/Settings.java

java/src/com/android/inputmethod/latin/SettingsValues.java

.../com/android/inputmethod/latin/UserHistoryDictionary.java

...id/inputmethod/latin/UserHistoryForgettingCurveUtils.java

java/src/com/android/inputmethod/latin/Utils.java