No point duplicating more generalized code that already interacts with the Android JobScheduler. So we should migrate our notification jobs to BackgroundTaskScheduler.

We should only ever use it on M+ though since our jobs need to execute immediately, and under the hood BackgroundTaskScheduler will use GcmNetworkManager pre-M in future, which does not allow immediate execution. 

To begin with we may want to just use it for N+ to match when we currently schedule a job rather than start a background service.