Override the access decision strategy instead of the manager

[aschempp]

Contao is decorating the Symfony access decision manager, because we require to use the priority strategy in our firewall, but we don't want to alter the strategy in a possible firewall outside of Contao. Unfortunately, Symfony does not allow to define the voter strategy per firewall.

Our decoration causes several issues, mostly described in symfony/symfony#54225. A quick fix was added in #7008.

Decorating the access decision manager causes all kind of issues in the profiler toolbar. We worked around some by decorating our own access decision manager with the Symfony TraceableAccessDecisionManager in debug mode. But Symfony also decorates all voters automatically to trace the voter definition, and these traced voters are only replaced on the original access decision manager. Our profiler therefore did not show all relevant information.

Because we only need to influence the strategy, we can simply decorate that instead of the full manager. Tracing and everything then works out of the box as usual in Symfony. We can even show the correct strategy in the profiler depending on the firewall context 🎉

@fritzmg I think that closes symfony/symfony#54225 (at least for us)
Also fixes #6988

[leofeyer]

Thank you @aschempp.