OMS: Fix bug where overlay settings wouldn't persist

The dead user check was using a List.removeAll() call to find
the subset of users that have been removed and need deleting
from the overlay settings. The target class, UserInfo, does not
implement equals(), therefore no UserInfos would be removed from
the set and all user IDs would be candidates for deleting.

This change avoids the UserInfo class entirely and manipulates userIds.

Bug: 36099320
Test: Pick a theme and reboot.
Test: Theme should still be applied on next boot.
Change-Id: I1ee57839515460bf578664dfe1bd67df7d10d041