Remove Users & profiles header for phones

In Settings/Users, remove "Users & profiles" header is no longer displayed
for phones. List of UserPreference is now explicitly sorted. Before, it
was relying on an overridden compareTo method in UserPreference, which
behavior was difficult to predict when instances of UserPreference were
mixed with other instances in the same group.

Bug:17630523
Change-Id: Iba1ef863c2cd4e8ca307ae0aa0877c5cd9e0973d