Allow 3P apps to provide structured data within AssistContent.

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