Guest user is last.

There is a bug where the guest user might get placed between two
non-guest users in cases where a non-guest user was created _after_ the
guest user was created. This is because in ag/20155790, we added sorting
by creation time but missed this requirement.

This CL fixes that.

Fix: 254258116
Test: This was done using TDD (test-driven development). The test was
first updated to fail, then the production code was changed to make the
test pass. Finally, I manually verified that a guest user, which I
created before a normal user, is displayed at the end of the user list
in both the full-screen user switcher and the popup version. Also
verified in the bouncer drop-down.

Change-Id: I0f4eaaddc4b264eb7457e0f731412f2cf93e2e8b