feat(app_check, android): add option to use FIREBASE_APP_CHECK_DEBUG_TOKEN env variable as debug token on Android

[xVemu]

Description

If you run your app, FIREBASE_APP_CHECK_DEBUG_TOKEN is present, and you're using debug provider, then it will use a token from env instead of generating one. It's useful in CI environments, where you don't have an option to copy token from logs and paste it to Firebase Console.

Related Issues

Partially fixes #11719

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Breaking Change

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