Track active freeform tasks

Track active freeform tasks and filter them from recent tasks in overview if
desktop mode is active.
A task is considered an active freeform task if it meets the following criteria:
- task is running
- task has been visible at least once when desktop mode was on

Bug: 244348395
Test: atest RecentTasksControllerTest
Change-Id: I10854b467615fc5bb257c1719d3887643d28fd52