Remove asset manifest

[biggs0125]

Description

Flutter no longer provides access to the AssetManifest.json file that google_fonts was depending on:
https://docs.flutter.dev/release/breaking-changes/asset-manifest-dot-json

Instead Flutter provides an API to access this same information.

This was causing "AssetManifest.json not found" errors and test failures in other repos like dart-pad which depend on this package.

Tests

I've updated the tests to mock out the AssetManager API. We're only using the listAssets API here so this is just a matter of returning the correct List of strings for each test.

Issues

Fixes # #661

Checklist

• I've reviewed the contribution guide.
• I've updated the package CHANGELOG.md

[parlough]

Thanks for updating this Nate!

I'm not sure if @guidezpl will be able to review this? @jwill perhaps you can help?