res_pjsip_sdp_rtp.c: Initial RTP inactivity check must consider the rtp_timeout setting.

res_pjsip_sdp_rtp.c: Initial RTP inactivity check must consider the rtp_timeout setting.

First rtp activity check was performed after 500ms regardless of the rtp_timeout setting. Having a call in ringing state for more than rtp_timeout and the first rtp package is received more than 500ms after sdp negotiation and before the rtp_timeout, erronously caused the call to be hungup. Changed to perform the first rtp inactivity check after the timeout setting preventing calls to be disconnected before the rtp_timeout has elapsed since sdp negotiation.

Fixes #710

(cherry picked from commit 9cf8678112083246d1aa15344f1508170a797f85)