Fix several issues with precreated users.

The feature is currently broken in master - so this CL fixes it.

1. Prevent UserManager from destroying storage for precreated users.
2. Modify UMS.getUserIds to exclude precreated users.
3. Remove pre-created users if the system has upgraded.
4. Read permissions during conversion to a "real" user. Permissions should have been granted during the pre-creation. If we cannot read permissions, re-grant them for the user.

Fixes: 143464654
Fixes: 143463955
Test: Repeated subsequent boots; observing logs; boot systrace; applied OTA, verified user cleanup
Change-Id: Ib25eac7c27ec3aea2b2a7eef040651bd06590a2e
(cherry picked from commit debb009a)