init: Queue Triggers instead of Actions

When init queues a trigger, it actually enqueues all of the Actions
that match with that given trigger.  This works currently because
all init scripts are loaded and therefore all Actions are available
before init starts queueing any triggers.

To support loading init scripts after init has started queueing
triggers, this change enqueues Trigger objects instead of their
matching Actions.  Each Trigger object then matches its associated
Actions during its execution.

Additionally, this makes a few cosmetic clean ups related to triggers.

Bug: 23186545
Change-Id: I5d177458e6df1c4b32b1072cf77e87ef952c87e4