messageTray: Remove dead code for updating the notification stack
messageTray: Remove an extraneous show

This means that the close button also does not have to be public.
messageTray: Clean up the code that animates the notification

Right now the code chooses to animate based on whether or not the
notification was "removed", which is quite a sketchy subject. For
now, add an additional case so that we don't animate when we transition
to the lock screen.
messageTray: Clean up _updateState for notifications

_updateState has a lot of variables that sort of gunk up the
code and make it more unreadable than need be. Clean up the logic
a lot by moving those variables into the places that they actually
matter, renaming them to remove prefixes, and remove some conditions
that are always met.
messageTray: Clean up _updateState for the summary box pointer

This code still isn't great, but it's an improvement over what we
had before.
messageTray: Don't close the tray when responding to a notification stack

This is against the designed behavior.
messageTray: Remove the stackedNotification tracking complexity

We only have one signal left to track, but since it's destroy it
doesn't matter if it's disconnected or not since it will eventually
be GC'd.
messageTray: Remove code that happens on notification destruction
messageTray: Use the BoxPointer APIs for hiding without animating