fix #6259: do not save cookie store with Google

[samueltardieu]

Keeping the cookie store separate from the preferences allows not to
backup it with the backup manager.

[pstorch]

Interesting. Didn't know about the concept of preferences backup via google. Had to look it up.
But the solution sounds plausible.

[pstorch]

Should we also reset the cookies on CredentialPreference.revokeAuthorization()?
The AbstractLogin class has a resetLoginStatus() method.

[Lineflyer]

As written in #6259 I suggest to put this to master only (and maybe also discuss the background and further steps).

[pstorch]

@Lineflyer the target branch of a PR can also be changed via the edit button on top of the page. Ok, this doesn't work in all cases, but most of the time it is not needed to close and open a new PR.

[Lineflyer]

Then lets @samueltardieu do the needful. :)

[samueltardieu]

Should we also reset the cookies on CredentialPreference.revokeAuthorization()?

Good idea, I'll do that in case of revocation of gc.com authorization.