Implement new 'autovalue' parameter property

Add an 'autovalue' parameter property to the TemplateData spec. And
implement it in the validation for the API and in the editor.

Also added tests to make sure all parameter attributes preserve
their values before and after parsing, including the 'autovalue'
parameter.

Bug: 51428
Change-Id: Iffb376a804d39388d2b5b6ea3583ef2a292eea41