Skip some duplicated tasks when booting from non-system user.

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

In the long term, refactoring the boot sequence on headless-system mode should avoid these
duplicated events, but for now, we need to tackle them case by case.

Test: manual verification # adding system.out statements on automotive

Bug: 139502237
Bug: 128904478
Bug: 138956267

Change-Id: Idd8beedf1ea034b47b9756b91404c5a043965c23