Fix issue #6745498: Cannot view consecutive event details from agenda view

- There was a long-standing bug when using FLAG_ACTIVITY_REORDER_TO_FRONT
where we could find and use an activity that is currently finishing.
- There was a recently introduced bug where activities being destroyed
would not be removed from the history stack at the time they are done
being destroyed, allowing the above bug to be exposed.
- Removing a task would not kill any processes associated with the app
that had a different name from the app itself.

Change-Id: I4401ab6d348a69e1ac4fb8f719d2c69d5a78e567