Introduce EMPTY_PREV_WORDS_INFO and BEGINNING_OF_SENTENCE.

...m/android/inputmethod/latin/ContactsBinaryDictionary.java

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

...rc/com/android/inputmethod/latin/RichInputConnection.java

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

.../com/android/inputmethod/latin/inputlogic/InputLogic.java

...m/android/inputmethod/latin/utils/LanguageModelParam.java

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

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

...hod/latin/personalization/UserHistoryDictionaryTests.java