Update stylus button press recognition in GestureDetector to use new APIs

Button press events now occur in onGenericMotionEvent. This CL update the
button press detection code in GestureDetector to recognize the gestures
within onGenericMotionEvent rather than in onTouchEvent.

Bug: 21148238
Change-Id: I6729668ed9bb600d8bf5e42e5ba9d8201b642d68
3 files changed