tmf: Cleanup Statistics View classes a bit

tmf: Cleanup Statistics View classes a bit

- Remove useless interfaces
- Change protected fields to private, make some of them static
- Use Immutable List/Set where applicable

Change-Id: Ia351e820ea6d5988074d227d369db9f821374026
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/26476
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>