tmf: Don't access the attribute tree directly

tmf: Don't access the attribute tree directly

but go through a new StateSystem.getAttributeTree() method that
wraps it. This way, a potential subclass only needs to override
that method to point to a different attribute tree.

Change-Id: I77b306f53095eb7b7423e10ee1ebb97c3ddaf6f5
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/10856
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bhufmann@gmail.com>
IP-Clean: Bernd Hufmann <bhufmann@gmail.com>