Bug 1061854 - Dismiss notification swiping up r=mhenretty

With the new ambient indicator, in order to dismiss a notification, a swipe up gesture is needed. All the swipe left code has been removed in this PR.
Well, it was done just for readability (of the code that uses it), and just in case at some point we change what 'current notification' means, but fair enough, happy to change it back to the if :)