Fixed guest check so it allows multiple pre-created users.

Also fixed shell cmd so it does not require name for pre-created users

Bug: 111451156
Bug: 132111956

Test: adb shell pm create-user --pre-create-only --guest # couple times

Change-Id: Ie256d55bdfe685f040d16223fcc70f2f2b7969da
(cherry picked from commit e35f5b01)