Bring back action_view lazy loading

04dfaf74eecca0745efa3e45636d76524380d6f6 brought a side-effect that it causes AV.on_load hook to be always kicked after the application initialization, and as a result, it causes performance regression in cases that ActionView isn't actually needed (e.g. booting up Rails console, running a model test, etc.)

So, here's another take for fixing https://github.com/rails/rails/issues/9461. Let's simply load AV::Base where it's needed.