NewAKString is effectively the default for any new IDL interface, so
let's mark this as the default behavior. It also makes it much easier to
figure out whatever interfaces are still left to port over to new AK
String.
This makes use of the new [UseNewAKString] extended attribute. Using
Vector storage will make it easier to make this interface into an IDL
iterable. It seems the reason it didn't use Vector originally was due
to awkward DeprecatedString -> String conversions.
