Improve getting namespaced class name.

Improvements over existing code:

- Currently if you pass `$viewClass` argument or have `$this->viewClass` set to `View` it skips calling `App::className()` which necessitates having a `use` statement for that class. This shouldn't be necessary. The method should have same behavior regardless of what the class is or how it's passed to it.
- The `pluginSplit()` call is not necessary since `App::className()` already does it internally.