MD Settings: user list fixes

This CL:
* Updates the API code to make better use of user_manager::User.
* Adds an isSUpervised property to usersPrivate::User.
* Hides the email address for supervised users and whitelisted users not on the device (where the name is unknown).
* Adds CrScrollableBehavior to the user list so that top and bottom borders for the list display properly.
* Adds a requestUpdateScroll method to CrScrollableBehavior so that borders are set properly when the list updates (in addition to when it scrolls).


Cr-Commit-Position: refs/heads/master@{#468360}
7 files changed