Return error code from isProvisioningAllowed

Added hidden pre condition codes for PO and DO provsioning.
Added hidden api checkProvisioningPreCondition, which returns codes
instead of boolean. Managed provisioning can use this to show
useful debug information and user facing error dialogs.

Test: All DevicePolicyManagerTest pass

Bug: 27467633
Change-Id: I7d2a79921bc3ac2e12d506629a35563fc7ff62bf