Refactor to start using JobScheduler instead of Service

[jwangenheim]

Background

Our EngineService is started in many different scenarios:

• When the user opens our app
• When Samsung Internet requests our filters file through our ContentBlockerContentProvider
• When ConnectivityChanged broadcast receiver is triggered

With that fix from #6389, the only change on behavior is that our app might stop checking for updates when the user intentionally kills the app. But once he goes through any of the scenarios above, the update check will resume.

For repetitive tasks in an Android app, we need to consider that activities and services can be terminated by the Android system to free up resources. Modern Android applications should use the JobScheduler API for those tasks. Apps can schedule jobs while letting the system optimize based on memory, power, and connectivity conditions.

What to change

Implement JobScheduler and use it for all tasks that are currently handles by the EngineService.

[jwangenheim]

Marked as duplicate. See #6454