tmf.core: making fInstance private

tmf.core: making fInstance private

Private members encapsulate data well in a class and are not used
anywhere else in the code so they can be easily modified or enhanced
without any side-effects.

Change-Id: Ida21a9a5c576f20ee2a5bee29b80ef02de9eb630
Signed-off-by: Wassim <wassim.nasrallah.36@gmail.com>
Reviewed-on: https://git.eclipse.org/r/75095
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>