tmf: Use location OS string for validation path during drag and drop

tmf: Use location OS string for validation path during drag and drop

The location URI path is not compatible with java.nio.file.Path in
Windows OS.

Change-Id: I80cf09ab54d8d3fffee23b6b3f692de15810eb56
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/32510
Tested-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>