Detect and notify about aborted tasks

[saeub]

Detect lifecycle events:

• Android: https://flutter.dev/docs/get-started/flutter-for/android-devs#how-do-i-listen-to-android-activity-lifecycle-events
• iOS: https://flutter.dev/docs/get-started/flutter-for/ios-devs#how-do-i-listen-to-ios-lifecycle-events

Add an API endpoint /task/{taskId}/abort.