Add gesture trail feedback.

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

...c/com/android/inputmethod/keyboard/LatinKeyboardView.java

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

.../android/inputmethod/keyboard/internal/GestureStroke.java

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