pcap: Fix null warning in ProtocolConversion

pcap: Fix null warning in ProtocolConversion

Rework the ProtocolConversion utility class to switch/case
through the existing values, instead of relying on
non-null-annotated methods.

Also updated the documentation.

Change-Id: I9de733029de08e976acca313a8ad4601d3db0a8b
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Signed-off-by: Vincent Perot <vincent.perot@ericsson.com>
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/31588
Tested-by: Hudson CI