Sample App for User Activity States

i.e. an app for tracking changes in user state, per: https://developer.android.com/health-and-fitness/guides/health-services/monitor-background#useractivityinfo

Start with the linked Android Health Services guide, especially the UserActivityInfo section, then inspect the repository's existing sample apps for structure and entry points. The issue does not name files, tests, or acceptance criteria, so confirm the supported user activity states and expected sample behavior with maintainers before implementation.

Written by the indexing model from the issue text.