Try to avoid overlapping job execution.

In certain situtations, when an app schedules a new job using the same
job ID as an already executing job, JobScheduler may start running the
new job before it has fully stopped the old job and cleaned up state.
This may cause some incorrect behavior within apps that don't properly
handle this possibility. To alleviate that, try to avoid executing the
new job until the old job has finished, and document the behavior as
much as possible.

We still continue to start TOP+EJs immediately so that developers can
operate with that guarantee.

Bug: 231148615
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/job
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/job
Test: atest CtsJobSchedulerTestCases
Change-Id: Ieedd7a9b91c350e0b835163fa9a73913d1edd345