feat(app_installations): add Pigeon support

[SelaseKay]

Description

Adds Pigeon HostApi bridging for firebase_app_installations on Android (Kotlin) and Apple (Swift/macOS), replacing the previous MethodChannel call sites for delete, getId, getToken, and registerIdChangeListener.

onIdChange continues to use per-app EventChannels (same pattern as App Check). Public Dart API and the web implementation are unchanged.

Verified locally with platform-interface HostApi unit tests and the package's existing unit tests. Ran melos run format-ci before opening this PR.

Related Issues

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