[application] Fix the remaining code pieces relying on single-app mode.
- Remove ApplicationService::GetActiveApplication and hence modify some of application tests accordingly
  (they use now ApplicationService::Launch API and work with the obtained Application instance).
- EventManager is considering Appplication's life-cycle.
- Clean-up
Merge pull request #1489 from pozdnyakov/Remove_GetActiveApplication

[application] Fix the remaining code pieces relying on single-app mode