Do not prevent default on touchstart

handleTouchStart_ used to call preventDefault to prevent "contextmenu" from being fired on "MsPointerDown" events. But calling preventDefault also prevents browser "click" events from being fired, which is a problem. This commit fixes it by adding a specific function for handling "MsPointerDown" events.

But stil, I don't think we should unconditionally call preventDefault on "MsPointerDown". This is too drastic. Calling preventDefault should probably be done at a higher level, where/when we know we handle this event ourselves and don't want the default behavior to trigger.