eflowRec: replace Decorator with WriteDecorHandle

Hello,

This MR is replacing a Decorator with a WriteDecorHandle(+Key). The main motivation is to suppress these warnings which appear once per event in the HLT:

02:41:36 copyAuxStoreThinned                                    8     0 WARNING AthContainers/src/copyAuxStoreThinned.cxx:134: unlocked decoration passPFTrackPresel (269) in object of type xAOD::TrackParticleAuxContainer_v5

Cheers, Bertrand