Decide the current user after PHASE_THIRD_PARTY boot phase

- Current user can change during the boot phase from other system services.
- If the current user has changed, and the system user is not started yet,
  crash the system as the system component starting other user should
  have started system user already.

Bug: 128871357
Test: Reboot and check if user switching happens in right order
Change-Id: I7e98e4a9f9f57612dca3052615689555cc965705