init: clean up DelayService()

ServiceList's services_update_finished flag was overlapped with the
global flag: is_default_mount_namespace_ready. Now DelayService() relies
on the is_default_mount_namespace_ready flag.

Add a service description with 'updatable' flag and invoke 'start
<name>' in 'on init' block (which comes before APEX activation).
See the log for "Cannot start an updatable service".

Bug: 293535323
Test: see the comment
Change-Id: I9341ba1a95d9b3b7c6081b530850d61f105f0a56