fix: stop using AssetManifests.json

[alestiago]

Description

We should stop using AssetManifest.json (deprecated) to find assets and instead rely on AssetManifest.bin.

From the Flutter documentation:

The flutter CLI tool generates a new file, AssetManifest.bin. This replaces AssetManifest.json. This file contains the same information as AssetManifest.json, but in a different format. If you need to read this file from code that is not part of a Flutter app, and therefore can't use the AssetManifest API, you can still parse the file yourself.

Additional Context

• https://docs.flutter.dev/release/breaking-changes/asset-manifest-dot-json#reading-asset-manifest-information-from-dart-code-outside-of-a-flutter-app