Add extra to control USER_SETUP_COMPLETE behavior.

For split-user cases it has been identified that provisioning
a profile-owner during user setup-wizard shouldn't cause
the setup-wizard to exit early generally. Adding this extra
allows a DPC to control whether the user can take further steps
in the setup-wizard after management provisioning completes.

Adding as a hidden extra since we don't expect this to be useful
until after N release.

Bug: 25858670
Change-Id: I599a5df4aef659769a6323402efe078d0d12d2ed