sys user split: fix primary flag bug.

When the end user first login as Guest, and the device rebooted during
the session (Guest user not removed), then the first user created
afterwards would not be correctly marked as primary.  We need to
explicitly looks through all current users to be able to tell for
sure.

Bug: 19913735
Change-Id: Iab1160c17c8dfdc054fd5dd136435fe2711dfa0c