After long update no exposure, additonal logs & potential fix (EXPSOUREAPP-4120) (#1818)

* Use startedAt as fallback when finishedAt ist null to avoid new download tasks get launched while a task is running

* Additional logging on DefaultDiagnosisKeyProvider

* work-runtime-ktx to latest stable

* better mapNotNull, adjusted Exception

* removed worker downgrade for separate PR

Co-authored-by: Ralf Gehrer <ralfgehrer@users.noreply.github.com>