Move recommended location for Flutter module in Angular

[redbrogdon]

Ideally, the compiled version of the Flutter module used with the Angular project would be maintained in the assets rather than the src folder. This is because Angular considers /src to be the root for source files it needs to examine and compile, while /assets is for those things that are to be included by not compiled.

We should update the README to present this as the happy path for doing the integration, and update the Angular app in the repo to use that approach.