[login_manager] Code to add the owner to the whitelist in a device policy

Today, every time the owner logs in, we ensure he is on the whitelist and
that we have him recorded in the signed settings store as the owner.

Add code that can do the same thing for a device policy.

TEST=unit tests.

Change-Id: I0073c7b0216879fb021f2319906ffd51bd54233f

Review URL: http://codereview.chromium.org/6815021
13 files changed