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.
Why is this in a separate function? We only use this in one place above, and moving it to a scope level function down here makes it a little less readable in my opinion. Also, it makes us have to assign a local variable to the `this` object, which we generally try to avoid these days in favor of binding.