Create general callback for changes to RecentTaskList

New callback called whenever a user visible task
is added or removed to the recents list. This results
in less work by updating only when recent task list
changes instead of listening for active task list
changes.

Test: atest RecentTasksTest

fixes: 111077107

Change-Id: I9acf13762d0c79bfde90b64fa5e0edaf882068cc