Add beginning of sentence information in PrevWordsInfo.

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

...putmethod/latin/RichInputConnectionAndTextRangeTests.java