In late August, Google announced limited support for app search for person content. In short, it allowed developers to integrate their apps to let Google index locally personal content within the app, to be surfaced on search on Android devices, no wifi needed
But when it launched, it launched with select Android apps. Now, any developer can add it to their Android apps.
Google said "we're inviting all Android developers to enable this functionality for their apps."
Starting with version 10.0, the Firebase App Indexing API on Android lets apps add their content to Google's on-device index in the background, and update it in real-time as users make changes in the app.
For more details on how to implement this, see the developer docs.
Forum discussion at Google+.