Move guest creation to background thread

Fix bug where phone could not unlock if guest user was being created in
the background.

This bug only affected devices with config_guestUserAutoCreated=true.

Test: In scheduleGuestCreation, replace createGuest() with
      Thread.sleep(20000). Ensure device can be unlocked within a few
      seconds after killing sysui process.
Bug: 194531015
Change-Id: Idc86a7afdb58a64baa3725d8aa56d7c93a409662