Delay cleaning user tasks until user is removed

Fixes #24301208 No recent apps shows when switching
between users.

Instead of cleaning up when stopping a user, we
should remove tasks when removing a user, since
recents tasks should be persisted across reboots.
Reboots are similar to stopping and starting users.

Change-Id: I9a250792077cca5f18ae1a10bc36f7b97e8ea867