Merge "Refactor drawing preview classes"

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

...inputmethod/keyboard/internal/AbstractDrawingPreview.java

...putmethod/keyboard/internal/DrawingPreviewPlacerView.java

.../keyboard/internal/GestureFloatingTextDrawingPreview.java

...method/keyboard/internal/GestureTrailsDrawingPreview.java

...thod/keyboard/internal/SlidingKeyInputDrawingPreview.java