IKernelAnalysisEventLayout: add fieldPrevPrio() method

IKernelAnalysisEventLayout: add fieldPrevPrio() method

Adds a fieldPrevPrio() method in the layout to access the
priority of the previous process. Up to now, we could access
the previous process' name, but its priority can be useful.

Change-Id: I52c9ae1f3ba377139702e8cd4a6aad75cac7757c
Signed-off-by: Raphaël Beamonte <raphael.beamonte@gmail.com>
Reviewed-on: https://git.eclipse.org/r/70503
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>