Have Special:CentralAutoLogin honor "Keep me logged in"

When the user checks "Keep me logged in", the cookies on the central
domain and any processed by Special:CentralAutoLogin should honor that.

Also, this fixes the bug where the central session data was being
copied instead of shared between wikis.

Bug: 51644
Change-Id: I7a41cb927d9a58668355ee62e621209327efce23
(cherry picked from commit 76012b7a39827c3da9adfebd36f3a66f36d36365)