init: trigger shutdown directly from builtins

Especially now that property_service is a thread, there may be some
delay between when init sets sys.powerctl and when the main thread of
init receives this and triggers shutdown.  It's possible that
outstanding init commands are run during this gap and that is not
desirable.

Instead, have builtins call TriggerShutdown() directly, so we can be
sure that the next action that init runs will be to shutdown the
device.

Test: reboot works
Test: reboot into recovery due to bad /data works
Change-Id: I26fb9f4f57f46c7451b8b58187138cfedd6fd9eb