Don't start first human user twice on headless-system mode.

It speeds up boot on automotive in about 25ms.

Bug: 128904478
Bug: 132111956

Test: manual verification
Test: atest CtsVoiceInteractionTestCases CtsAssistTestCases # on walleye and automotive

Merged-In: I37cdca09b5280193a07ed9951f2e386ed3cd2299
Change-Id: I37cdca09b5280193a07ed9951f2e386ed3cd2299
(cherry picked from commit 206c09b55331440ded234349dbbe9716b91a095a)