Ensuring that there are per-user last stack active times.

- This fixes an issue where the last stack active time would be clobbered
  when switching between users.  With the policy in the phone/stack
  recents, this is fine, but with the grid recents, it no longer only
  applies when out of the historical window, so it is always wrong (it
  would normally be wrong if switching back from another user after the
  historical time of six hours).

  This CL will migrate the last stack active time to a per-user secure
  setting, which will be used going forward.

Bug: 35375206
Test: On the Ryu, launch some tasks, switch users, launch more tasks, and
      return to the original user

Change-Id: I9941526de5d1dd52d1f9003e795995389064b19d