power: let init manage the service directly

We don't need minijail to stick around just to waitpid() us when init
can do it for us.  Pass -i to tell minijail to exit immediately.

BUG=None
TEST=restart powerd works, and minijail parent no longer exists

Change-Id: I88935018a501a224698f528938c7f2de7fced77f
Reviewed-on: https://chromium-review.googlesource.com/393086
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>