VT: Enhancements on RTP packetizer.

  1) Apply discounted bitrate for initial RTP case
    [Problem] There is a possibility that sending bitrate
      overs the AS value, if there is no TMMBR signal.
    [Solution] Apply the discounting also in inital bitrate.

  2) Correction of UDP fragmentation.
    [Problem] Packets can not be sent to remote in some network
      environment(R&S ATT TE). Packet should be fragmented up to
      1280bytes in case of IPv6.
    [Solution] make a condition to build a packet up to 1280bytes.

  3) Correct a condition to check last of the FU-A.
    [Problem] Marker bit (of RTP header) was 0 even it was last FU-A packet.
      A RTP buffer size checked with RTP header extension, but it filled
      without header extension size.
    [Solution] Correct a condition to set market bit as 1.

Bug: 165061754
Change-Id: Ic70b06b330394b40b1c756aba85e175fba178b8d
Signed-off-by: Byeongjo Park <bjo.park@samsung.com>