tmf.core: Partial ss API should not expose restricted classes

tmf.core: Partial ss API should not expose restricted classes

Since the callers of the partial state system do not have access to the
StateSystem class, it should not be required. It is replace by an
internal validation of the input.

Change-Id: Id2c345fa36ce048990419feba4898b64e7a14823
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/90308
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Alexandre Montplaisir <alexmonthy@efficios.com>