Allow 3P apps to provide structured data within AssistContent.

For providing structured metadata to the assistant, we've decided to
standardize on JSON-LD (met with Paige/Ulas/Fabian), and decided the
data to be specified via:

AssistContent.setStructuredData(String structuredDataJson)

This method can be documented for 3P developers as allowing developers
to provide metadata to the assistant with the JSON-LD standard.

This would be centered more about content displayed on the screen,
rather than actions based, so we added it to the AssistContent class

Updated doc: go/setStructuredData

Change-Id: I816f69fd6896df822e30d6c71d840bd01f47e544