Setup mirroring after session configuration so output respects mirror setting

There's an apparent bug where if you use mirroring (usually for the front-facing camera), the output is mirrored correctly on the preview layer but not in the actual capture (for both photo and video). The issue stems from replacing the output during _setupSession without re-running the mirror setup code, so if you're switching devices or just using the auto-mirror mode you can end up with a busted output. This change should resolve the issues.