[starnix] Move task priority into scheduler policy.

Creates a single location from which to update Zircon
thread profiles.

Real-time schedulers are now inherited by a child
process like the nice value, which gets another
bionic test case to pass. However these schedulers
still do not have any practical effect on how Zircon
schedules a Starnix-based task.

Bug: b/297438302
Change-Id: I908354cdc525ad0f764842590a0e33ee11327489
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/908934
Fuchsia-Auto-Submit: Adam Perry <adamperry@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Adam Barth <abarth@google.com>