turn on send_silence_when_idle whenever we engage srtp

src/switch_rtp.c

@@ -1688,6 +1688,8 @@ SWITCH_DECLARE(switch_status_t) switch_rtp_add_crypto_key(switch_rtp_t *rtp_sess
 
 	memset(policy, 0, sizeof(*policy));
 
+	switch_channel_set_variable(channel, "send_silence_when_idle", "true");
+
 	switch (crypto_key->type) {
 	case AES_CM_128_HMAC_SHA1_80:
 		crypto_policy_set_aes_cm_128_hmac_sha1_80(&policy->rtp);