res_rtp_asterisk.c: Increase strictrtp learning timeout time.

More complicated direct media reinvite negotiations can result in longer
delays before direct media flows.  The strictrtp learning timeout time
was too short.  One log showed that the first RTP packet came in just
after three seconds.

* Increase the strictrtp learning timeout time from 1.5 to 5 seconds.

ASTERISK-27453

Change-Id: Ic5e711164cbb91b4d1c1e40c83697755640f138c