upgrade appdied transition trigger to task if relevant

When an app is killed, we send a transition request for it but
with the trigger at activity-level. However, the handleAppDied
logic immediately detaches the Task which means Shell can't
prepare for that (particularly split-screen).

This change will upgrade the trigger to task-level if the task
is going to be removed.

Bug: 251459369
Test: atest EnterSplitScreenFromOverview
Change-Id: Iaf901cb7db41a23a482b67801172053095c8dd47