Let users type their username for 1 s during registration before barking

250 ms are not enough to finish typing your username and the warnings on
the username not being available are fully relevant only when you're done
typing, while being actively harmful if you're still typing an actually
valid name.

We should wait as much as possible, as long as most users are given the
warning before submitting the form.

Bug: 71744
Change-Id: I646d76d2b29b8049c4464f156320d32ea4bd207a