Show removed user's data usage under its correct item

Users for which UserManager.getUserInfo returned null were shown as
unnamed users on Settings > Data Usage. This behavior has now changed
to show their data usage under the same item as UID_REMOVED.

Bug: 18174376
Change-Id: I8d172647edb51f7a277b4b714e9b7f3a0b5f3154
(cherry picked from commit cd9b1507)