video_rtp_session: skip packets on sender restart

When using nvenc and the media sender gets restarted, the peer's srtp
decryption breaks and only gets set up correctly again after a pause and
play.

This issue was traced back to the RTP sequence numbers. Skipping a few
numbers before reinitializing the sender works.

Change-Id: Ic93f1368fc7eb01b3c57341f31d3832c9f2f2c1f