Fix location of accessors in ACTS MTJ backends

Discovered in: !63678 (merged) In TrackMeasurement the accessors were needed in the template API of the EDM class and thus exposed publicly in xAOD namespace. This is bad and this MR fixes it. One clarification from @akraszna - the other classes TrakJacobian & TrackParameters that were mentioned have accessors defined as statics source file. DO you recommend to make them still private members?

Reviewers (first commit is just formatting-ident fix, no need to spend time on that)