TMF: Change API for synchronization files' folder

Some clients of experiment need a relative path, others need the absolute
path, the getSynchronizationFolder method is unified and the constant becomes
private.

Change-Id: I02688b55ec047820fbe756a269cf4bde3721a9b7
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/33133
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Hudson CI