AthContainers: Fix potential decoration race.

When we do a deep copy, we check that a variable is not a decoration before copying it. This was done by fetching a copy of the set of variables followed by a copy of the set of decorations. Then we iterate through the variables, testing if each is a decoration.

However, when we add a decoration, we were updating the set of variables before the set of decorations. It was thus in principle possible that we would miss the fact that a variable was a decoration when doing the deep copy.

I doubt that this would be an issue in practice, but to be sure, change so that we update the decoration set before the variable set. Also, when we do the copy, it is not actually necessary to copy the decoration set --- it is safe to just access it by reference.

c.f. ATEAM-1086