fix(firebase_app_check): Add play integrity check to app_check activation

[the-recondite]

Description

This PR adds the play integrity check to the already present SafetyNet app check when activating firebase app_check. Previously, only the SafetyNetAppCheck was being initialized. This PR was created to fix https://github.com/firebase/flutterfire/issues/9178. We need the play integrity check to make sure the installation of our app is legit.

Related Issues

closes https://github.com/firebase/flutterfire/issues/9178

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• [x] All existing and new tests are passing.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [x] The analyzer (melos run analyze) does not report any problems on my PR.
• [x] I read and followed the Flutter Style Guide.
• [x] I signed the CLA.
• [x] I am willing to follow up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• [ ] Yes, this is a breaking change.
• [x] No, this is not a breaking change.

[AngryVelociraptor]

Adding my support for this issue to be resolved, I was confused today when I learned that flutterfire only supports a deprecated API for android app check. Don't forget to update this page.

[russellwheatley]

Hey @the-recondite, thanks for the PR. Hope you don't mind, but we favored a different approach: https://github.com/firebase/flutterfire/pull/9646 😄