init: make triggering shutdown from vendor_init better

Previously, we assumed that TriggerShutdown() should never be called
from vendor_init and used property service as a back up in case it
ever did.  We have since then found out that vendor_init may indeed
call TriggerShutdown() and we want to make it just as strict as it is
in init, wherein it will immediately start the shutdown sequence
without executing any further commands.

Test: init unit tests, trigger shuttdown from init and vendor_init
Change-Id: I1f44dae801a28269eb8127879a8b7d6adff6f353