Fixing crash when dumping secondary SystemUI user service.

- Secondary user’s SystemUI process is initialized with 
  SERVICES_PER_USER, which is a strict subset of SERVICES, but mServices
  is a fixed size array that does not change, so we need to add a null
  check when iterating through the current list of services for the 
  non-primary user.

Bug: 28153575
Change-Id: I0a6b4726e82f2efddda358a835b1ef3d9f165375