Do not skip user starting for user 0

- USER_SYSTEM is in special state and startUser call is skipped without changing
  it into running state.
- Force the whole process for user 0 while non-user 0 keep the same behavior
  as it should not happen for those users.

Bug: 124460424
Test: Test system bootup (for 1st boot and subsequent boot)

Change-Id: I7adc6766c0ac33f766e7ca8769f555973f533ba3