srtp: fix decrypt errors

Before this patch when the the sender is restarted, we get the last sequence number from media_encoder that is refreshed on each frame only. But the sequence number continue to be incremented on each RTP packet sent, this result in rollover counter (ROC) desynchronisation in srtp context because the largest sequence number sent is now superior to sequence number that is supposed to occur only when sequence number overflow 65535.

With the patch when the sender is restarted, we keep the last sequence number from socket_pair (RTP level)

Change-Id: I531e3d0a073c251c78bbf9f0ffc702aafaf6ccc8