(bug 44775) Don't pre-fill username when creating.

In "Create account" (signup) mode, never auto-populate the username.
Pre-filling with the name of the user who last logged in is wrong (bug
44775), and if a logged-in user is creating a new account then
pre-filling with her own username isn't helpful.

Change-Id: I1bb641b2186a8c8dc92be0c7b5de2e9069dd31ed
Merge "(bug 44775) Don't pre-fill username when creating."