Add multiple gesture preview trails animation

java/res/values/attrs.xml

java/res/values/config.xml

java/res/values/styles.xml

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

...id/inputmethod/keyboard/internal/GesturePreviewTrail.java

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

...thod/keyboard/internal/GestureStrokeWithPreviewTrail.java

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