Add unit tests and fix implemention accordingly

* Add unit tests for all types of invalid input we check for.
* Add unit tests for all types of input we expand or otherwise
  normalise.

* Implement InterfaceText expansion/normalisation.
* Fix bug that caused a string value in the root description property
  to be considered invalid (it only accepted an object, it should
  accept both).

Change-Id: I5a15080f1f924451a9dde8af96ea2922011981ec
Merge "Add unit tests and fix implemention accordingly"