Don't back off for first few system stops.

System requested stops theoretically have no reason to trigger an
immediate backoff because there's no indication the backoff will help
improve success rate, so don't back off a job for the first few system
stops.

Bug: 282784507
Test: atest FrameworksMockingServicesTests:JobSchedulerServiceTest
Change-Id: If30edd312e17a6baba05654075e7ed21ace6338b