[unified_analytics] Create class NoOpAnalytics implements Analytics

[christopherfujino]

To un-block rolling flutter/flutter#124015 internally, I had to write a class NoOpAnalytics implements Analytics from this package. However, this will cause problems the next time this interface changes when we try to roll package:unified_analytics internally. Thus, the class should be implemented in this package.

Also, the type ToolInfo should be exported from this package, since it is returned from a public method on Analytics.