If hometask is last task to be dismissed, show empty recents instead of home screen

- http://ag/16221737 ensure homescren stub is always added if tasks hasn't been loaded in RecentsView. This introduced an edge case that after dismissing home task view, no task remains in overview and it get dismissed
- The fix is to show empty recents instead of home screen if last task dismissed is a home task (showing empty recents isn't always correct, as there might be unloaded tasks, but it's better than dismissing overview)
- Also tuned the timing of when mLoadPlanEverApplied is set

Fix: 206462357
Test: FallbackRecentsTest.goToOverviewFromHome
Change-Id: I0f395639f57a1a57fddf750623a8a74b8ede8555