Fix EJ syncs.

1. Only downgrade an EJ that doesn't have the IGNORE_BACKOFF extra when
   we are actually attempting to back off the job.
2. Allow EJs to have the FLAG_EXEMPT_FROM_APP_STANDBY flag since that
   can only be set by the system and won't cause issues.

Also propagate the scheduleEjAsRegularJob bit through the job extras to
help with debugging.

Bug: 178852366
Test: atest ContentResolverTest (all)
Test: atest CtsJobSchedulerTestCases:JobInfoTest
Test: atest CtsSyncManagerTest
Test: atest SyncOperationTest
Test: atest SyncManagerTest
Test: atest SyncRequestTest
Change-Id: I133d089a4d770a3cc37df72f8b38d234b24077df