ctf: Simplify StreamInputPacketReader

ctf: Simplify StreamInputPacketReader

- Make all internal structures final, initialize them in
  the constructor.
- Remove all the public getters and setters for those, they
  are only used internally.
- Replace a couple more null checks by "instanceof" checks.
  (previously, if a field name was present but not of the
  right type, it would throw a ClassCastException)

Change-Id: If4376a6310b48737a45892399dbb935e647ef0d8
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/7241
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>