Add Root#paramOrder to spec and implemention thereof

* Detail the new property in the spec.
* The property is optional (as all new properties have to be
  for backwards compatibility).
* Validate value if present, and for normalisation fill it
  with inferred order from the JSON parser. This means the API
  output will always contain #paramOrder, however the strict
  specification for it is optional and users must still account
  for absence of this property.

Bug: 53608
Change-Id: I7bcd7c9146f5ae75c4bad22b0a9cd4400f196c8c
Merge "Add Root#paramOrder to spec and implemention thereof"