SessionManager: Kill getPersistedSessionId()

SessionManager: Kill getPersistedSessionId()

It's not guaranteed that loadSessionFromStore() will succeed after
whatever alterations the SessionProvider might have made later in the
request.

So instead, let's make a new global object that stores the SessionId
of the persistent session that was loaded during Setup.php, if any. Then
we can check that when we need to know whether the session was
persisted.

Bug: T124468
Change-Id: I1e8e616c83b16aadd86b0a0a40826d40f6e8abe4