[JobQueue] Some tweaks to reduce claimRandom() retries.

* Use a small OFFSET to avoid excess claim retries for large queues.
* Use job_random to avoid excess claim retries for small queues.

Change-Id: I7da20a6684902122c7bc82b9633eab4bc7aecb89
Merge "[JobQueue] Some tweaks to reduce claimRandom() retries."