Do not delete task snapshot files when user stopped.

While pause a user, the tasks of the user will be removed from
recents. But there should just release memory vs delete snapshot files
when unloadUserDataFromMemoryLocked called. So the snapshot can be
reload from disk when user unlocked again.

Bug: 366374788
Flag: EXEMPT bugfix
Test: turn off device while some tasks of second user are exist, turn
on device, verify those tasks of second user shown on recents.
Stop second user without launch any task, verify no tasks of second
user shown on recents.
Enable second user again, verify those tasks can shown on recents.
Test: atest RecentTasksTest
Test: follow issue description.

Change-Id: I9a98339eaf6bd1a171aced598f61396eea7f8af2