b/3302354 Always pass multi-touch events into WebKit, then handle them in...

b/3302354 Always pass multi-touch events into WebKit, then handle them in WebView if preventDefault is not set.
WebViewCore.java: add a MotionEvent field into TouchEventData for multi-touch so the extra data such as pressure, size can be preserved.
WebView.java:
  1. Extracted the common code for multi-touch and single touch to a new private function: handleTouchEventCommon()
  2. Extracted the multi-touch code, including zooming and panning, to a new private function: handleMultiTouchInWebView()
  3. In onTouchEvent(), single touch events keep the code path, but multi-touch events are always passed to Webkit first.
  4. In Privatehandler.handleMessage(), if WebKit didn't consume the multi-touch event, the private handler calls the handleMultiTouchInWebView() to zoom and pan.

Change-Id: I3eba8812bd7cf6a298ff12e0b66223dae6725060