InDetTrackPerfMon: fixed clang warning, plus minor code refactoring

1. Clang generated a warning on the assignment operator for the TrackAnalysisCollections class: "explicitly defaulted copy assignment operator is implicitly deleted" because the base class AthMessaging has an inaccessible copy assignment operator. Marked the assignment operator as deleted

2. xxxTrackContainer data members of TrackAnalysisCollections were left in the public section. Moved them to private and added m_ prefix to conform with the ATLAS C++ coding guidelines

CC @maparo the author of !67568 (merged) which generated these clang warnings