SchedSwitchHandler: update the priority of the former process

SchedSwitchHandler: update the priority of the former process

Update the SchedSwitchHandler class to update the priority of
the former process as well as the one of the new one. This
allows to keep a 'sort of' up to date priority in the absence
of other events updating it.

Change-Id: Ie0da99274b9c932a0784dbd737d3279f2cfa6165
Signed-off-by: Raphaël Beamonte <raphael.beamonte@gmail.com>
Reviewed-on: https://git.eclipse.org/r/70504
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Tested-by: Alexandre Montplaisir <alexmonthy@efficios.com>