Don't do central login if not POSTed

Only do the login redirect through the central login server in the
UserLoginComplete hook if the request was a POST.

Bug: 69834
Change-Id: I99e9981aa06e9d0ae7adfe692c35687dbebab315
(cherry picked from commit 402867dc08008f7515c22d34b2f15a64bab80473)