Need attribute to track downloads in the application schema

The application schema provides attributes to track application installs, launches, closes, etc. but nowhere to track downloads. Downloads seems like a natural iteration of the application schema.

## What are the schemas that are affected by the issue
/context/application

## What are examples of products that are impacted by the issue
unknown

Open the /context/application schema and review its existing attributes for installs, launches, and closes. Add an attribute for download tracking consistent with that schema, then verify the application schema represents downloads.

Written by the indexing model from the issue text.