FlavorTagDiscriminants: Clean-up WriteDecorHandleKey declaration syntax

Some standardisation of how SG::WriteDecorHandleKeys are declared (and configured) in the Algorithms in the FlavorTagDiscriminants package. This is intended to make it simpler to update DecorHandleKeys consistently. It also avoids altering the values of configurable properties during initialize(), meaning that the values set in the configuration reflect what will actually be used in the event loop.