Change notification swipe-to-clear to use velocity

Rather than using an unscaled pixel value(!) for swipe detection, rely on the
framework's definition of a fling instead. Also use standard platform
animations to do the sliding instead of one that just "looks right"
which also makes some code redundant.

Change-Id: I22bfb408697ce6c502bbd6cde9cd0a9ad69bcc09