fix(android): implement App Check to protect APIs from being abused

[Chris1320]

When we call the API directly from a mobile or web app, the API is vulnerable to abuse by unauthorized clients. We can use Firebase App Check to verify that all incoming API calls are from our actual app.

[Chris1320]

More info: https://firebase.google.com/docs/ai-logic/app-check?authuser=0&api=dev