Fix a bug where context menu keeps popping up as we move cursor.

Previously, since there was not a clear code path for TouchEvents for
mEmptyView, we wrote one just for this specific class. However now with
everything in one class (ListeningGestureDetector), it makes sense to
receive events and just delegate them to UserInputHandler#onTouchEvent.

Bug: 30781381
Change-Id: Ied38fa92bbe3a2b12bba79a5cce5760eb73d1127