[wizard] Show warning on password page if a password is already set

[Michael Terry]

For designers and testers, if they re-flash without wiping, they are presented with set-your-password pages that actually won't do anything. Which is very confusing.

(This is because the wizard can only set a password if none is already set. If a password already exists, then it needs the previous password to set it successfully. But the UI doesn't bother prompting for it, since it's designed for OOBE when no password exists.)

We should not show the set-a-password screen, but instead show a warning saying "we would show you this, but..." or something (which is nicer than just skipping it, which would make a tester think it's broken).

This would just be for testers. But it would reduce a lot of confusion and some erroneous bug reports.