[kernel][object] Add TimerSlack to JobPolicy and pass explicitly

Introduce TimerSlack to encapsulate timer slack.

Extend JobPolicy with TimerSlack

Plumb TimerSlack through from syscall layer to timer subsystem.

No behavior change. A future CL will:
- provide a mechanism for setting the timer slack in job policy
- integrate job policy slack with timers

Bug: ZX-931 #comment Add TimerSlack to JobPolicy
Test: booted, ran "k ut job_policy" and runtests on qemu and vim2
Change-Id: I1f88533563280c93a15d24c25bed936d04e903ad