Don't send HOME intent twice when booting from non-system user.

When Automotive boots, CarServiceHelperService "restarts" the system user in the background, so it
can be unlocked. Consequently, some boot tasks will be executed twice (triggered from
UserController.startUser() and ActivityManager.systemReady()).

Test: manual verification
Bug: 139502237

Merged-In: Idd8beedf1ea034b47b9756b91404c5a043965c23
Change-Id: Idd8beedf1ea034b47b9756b91404c5a043965c23