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

Partial fix for: #11719

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[Lyokone]

Thanks for the PR. would you be able to bring this to iOS as well?

[xVemu]

After time, I think a better option is to let developers pass debugToken to FirebaseAppCheck.instance.activate function. They could read token from env by themselves or hard code it for debugging purpose. I've done some work on debug-token branch. Currently, it works well on Android, but not on iOS. I will try to work on that in near feature.

[xVemu]

I've created new PR with mentioned change #13101