xAODTracking, fix 1st the trivial warnings from THREAD_SAFETY_CHECKER ,ATLASRECTS-3315

Mentioning @and @akraszna that might be looking also to these.

Anyhow, the THREAD_SAFETY_CHECKER gives a lot of warnings on this package. It will probably take another 2 MRs to figure out the best solutions for the 3 mutable (1 NeutralParameters 2 Vertex, and the 2 static in the NeutralParameter not obvious especially for the DefiningParameter methods of the Neutal ... )

This fixes the trivial ones --> static Accessor , static const Accessor so to get them out of the way

Mentioning @goetz @emoyse @mhodgkin in case they know if someone else is looking on this or have ideas