Use TimeUnit instead of DateUtils for readability

...android/inputmethod/dictionarypack/DictionaryService.java

.../android/inputmethod/latin/utils/DictionaryInfoUtils.java

...utmethod/latin/utils/UserHistoryForgettingCurveUtils.java

java/src/com/android/inputmethod/research/ResearchLog.java

.../src/com/android/inputmethod/research/ResearchLogger.java

java/src/com/android/inputmethod/research/Statistics.java

...src/com/android/inputmethod/research/UploaderService.java