cros: Use ScreenManager as a component instead of deriving WizardController from it

This makes it easier/cleaner to implement CoreOobeView::Delegate on ScreenManager in a follow-up CL.

BUG=672142

Review-Url: https://codereview.chromium.org/2739073002
Cr-Commit-Position: refs/heads/master@{#456228}
12 files changed