QA: sane preferences handling

Please don't do Preferences/reset-all on shared test users, it
messes up other browser tests in beta labs.

In this case, it makes more sense to explicitly set the preference
in the Background, since all the tests expect it set that way.

This is Bug 70303

Change-Id: Id4201fc71a98c0b62690696b02bbc58f307c9c0d