Add an auto complete's threshold option.

java/res/values/config.xml

java/res/values/strings.xml

java/res/xml/prefs.xml

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

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

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

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

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