[firebase-appcheck-debug] Print debug token on every provider initialization instead of only when generated

### Environment
- **Firebase Component:** App Check Debug (`firebase-appcheck-debug`)

### Problem Description
Currently, `DebugAppCheckProvider` logs the debug secret (`Log.d(TAG, "Enter this debug secret into the allow list in the Firebase Console for your project: ...")`) when generating a new debug secret during initialization.

This behavior has two major drawbacks:
1. **Not printed on subsequent initializations:** If the secret was already generated on a previous run, or if logcat was cleared, subsequent app initializations / runs do not print the debug secret if it was only logged during the single initial generation event or if custom secret providers are used.
2. **Custom Secret Providers:** When an `InternalDebugSecretProvider` is configured, `determineDebugSecret()` is skipped entirely, meaning the debug token is never printed to logcat upon initialization.

### Proposed Solution
Update `DebugAppCheckProvider` so that every time the provider is initialized (including when an existing token is loaded from storage or provided via `InternalDebugSecretProvider`), the debug secret is printed to logcat.

Start with DebugAppCheckProvider and trace its initialization path, including determineDebugSecret() and the InternalDebugSecretProvider case. Check the generated, stored, and custom-provider flows using logcat; done means the debug secret is printed on every provider initialization.

Written by the indexing model from the issue text.