Add advanced options to turn on/off the gesture trail and text preview

java/res/values/strings.xml

java/res/xml/prefs.xml

java/src/com/android/inputmethod/keyboard/KeyboardView.java

...rc/com/android/inputmethod/keyboard/MainKeyboardView.java

...roid/inputmethod/keyboard/internal/PreviewPlacerView.java

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

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

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