[Blazor] Avoid redundantly qualifying event callback type checks

[javiercn]

There were a couple of issues here:

• The code that parsed EventCallback type arguments didn't account for global::.
• Once that got fixed, the EventCallback code wasn't distinguishing between explicit types and implicit/inferred types.
• For inferred types, there were two cases:
  • One where the type was replaced by a user defined type. In this case, we need to call CreateEventCallback<Type>
  • Two where the type was not replaced and needed to be inferred. In this case we need to call CreateEventCallback because the argument is being inferred using CreateInferredEventCallback

The change uses properties to track whether the user provided a type explicitly in code and whether we were able to replace the generic type properties in EventCallback and uses those facts to influence code generation.

[TanayParikh]

LGTM, just some clarifying questions for my understanding.