Avoid session double-start in Setup.php

In PHP before 7.3, the double start doesn't really matter: session_id()
changes the ID even if it was already started, and the warning from
session_start() can just be ignored. Which is what we did.

In PHP 7.3, now session_id() also warns and no longer changes the ID. To
preserve the previous behavior, we'll need to explicitly close the old
session and open the new one.

Bug: T213489
Change-Id: I02a5be1c3adb326927c156fdd00663bccee37477