Fix serial number assignment for new user on a freshly wiped device

Bug: 7280093

User serial number was not being bumped up from 0 on a freshly wiped device
due to the userlist not existing. This was resulting in the first 2 users
getting the same serial number, messing up C2DM message delivery. This issue
doesn't show up if you boot the device before adding a user, so it wasn't
discovered earlier.

Change-Id: I8a5d99b9ad2ccfb66a16cacac9548ba53f0be387