init: Add a way to class_restart only enabled services.

class_restart accidentally restarts disabled services. Changing this
behavior is risky as it could break compatibility. Instead, add an
"--only-enabled" argument to class_restart to opt-in to the new
functionality. This syntax is backward compatible, as previously only a
1-argument form was accepted.

Bug: 190065372
Bug: 198105685
Test: add a class_restart action and a disabled service, make sure
      service is not restarted.
Change-Id: Idb08779de7ac7a21e23f8b8a3276bd5a66a43299