Don't reuse userids that are being cleaned up on boot

During the current session, if we're cleaning up some
old users, mark them as being in the remove pile so
that the userIds are not reused right away. Otherwise
there's a potential for confusing the file-based crypto
layer.

Bug: 29635364
Change-Id: I57a868fbe6c58a02ffcea63809d25eba01360e1e