Remove transition / dyntransition from unconfined

Require all domain transitions or dyntransitions to be
explicitly specified in SELinux policy.

healthd: Remove healthd_exec / init_daemon_domain().
Healthd lives on the rootfs and has no unique file type.
It should be treated consistent with other similar domains.

Change-Id: Ief3c1167379cfb5383073fa33c9a95710a883b29