Allowing Jobs with no constraints.

Is JobScheduler a database now? Yes, yes it is.

The decision to now allow jobs with no constraints was made before
background restrictions were introduced. Now, with background restrictions,
it doesn't make as much as sense to have this policy.

Bug: 118636037
Test: Try to schedule a job with no constraints
Change-Id: Idfda95b415ae094a1ca5373f950b4a09c685b64f