Fix import formats of Firebase apps

[modular-magician]

• Use snake_case app_id
• Addresses an old issue with google_firebase_webapp import

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes). Field names in the import_format should be in snake case #14606 Clarify Terraform Import for google_firebase_web_app #7907
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

firebase: added additional import formats for app resources, removed the need to supply duplicate projects in some cases

Derived from GoogleCloudPlatform/magic-modules#7959

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