More small fixes/adjustments to job scheduler.

- JobServiceEngine now takes a concrete Service instead of
generic Context in its constructor, since it really must be
associated with a real Service.

- Expand documentation of how dequeueWork() operates.

- Fix some job scheduler implementation to hopefully actually
match the docs: transfer remaining executing work to the new
job, and actually correctly transfer state from old and new
jobs if we are rescheduling due to a true return from onStopJob().

Test: bit CtsJobSchedulerTestCases:*
Change-Id: Ia66797049883eefb566264f930070afb69d469b1