Rework backoff policy calculation.

Don't penalize jobs in the backoff calculation when they were stopped
for reasons that were likely outside of the app's control (eg. the
device entering Doze or battery saver or constraints no longer being
satisfied). Jobs calling jobFinished() on their own or timing out are
both deemed as being within the app's control.

Bug: 209979687
Test: atest FrameworksMockingServicesTests:JobSchedulerServiceTest
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/job
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/job
Change-Id: I3cffd676c81cb461df39e68999b1a97cf4d5521a