Add a dedicated method to control time in native code.

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

...ve/jni/com_android_inputmethod_latin_BinaryDictionary.cpp

...mpl/dictionary/structure/v4/ver4_patricia_trie_policy.cpp

...yimpl/dictionary/structure/v4/ver4_patricia_trie_policy.h

...roid/inputmethod/latin/BinaryDictionaryDecayingTests.java

.../com/android/inputmethod/latin/BinaryDictionaryTests.java