Disallow starting new system services after apex services are started.

Apex services must be the last category of services to start. No other
service must be starting after this point. This is to prevent
unnecessary stability issues when these apexes are updated outside of
OTA; and to avoid breaking dependencies from system into apexes.

Bug: 192880996
Test: presubmit
Change-Id: I42a5a1745c7fc0784fa2fef554c3d6439fd4c7fb