Don't retrigger bootstat during userspace reboot.

Userspace reboot can be treated as extension of runtime restart: instead
of restarting just system_server, it restarts all post-data services.

Test: adb reboot userspace
Test: checked bootstat didn't run again.
Test: system/core/bootstat/boot_reason_test.sh
Bug: 135984674
Change-Id: Ieda4b201f9b9294a3a54b7f0118ee769f5dd2065