- Introduced new config variables 'opus_samplerate', 'opus_stereo',

and 'opus_sprop_stereo'.
- Set 'opus_samplerate' to 16000 and 'opus[_sprop]_stereo' to "no" in order
  to support acoustic echo cancellation also when opus codec is in use.
- Renamed 'prefer_ipv6' config variable to 'net_prefer_ipv6'.

app/src/main/kotlin/com/tutpro/baresip/ConfigActivity.kt

@@ -52,7 +52,7 @@ class ConfigActivity : AppCompatActivity() {
         listenAddr.setText(oldListenAddr)
 
         preferIPv6 = findViewById(R.id.PreferIPv6) as CheckBox
-        val piCv = Config.variable("prefer_ipv6")
+        val piCv = Config.variable("net_prefer_ipv6")
         oldPreferIPv6 = if (piCv.size == 0) "no" else piCv[0]
         preferIPv6.isChecked = oldPreferIPv6 == "yes"
 
@@ -154,7 +154,7 @@ class ConfigActivity : AppCompatActivity() {
             var preferIPv6String = "no"
             if (preferIPv6.isChecked) preferIPv6String = "yes"
             if (oldPreferIPv6 != preferIPv6String) {
-                Config.replace("prefer_ipv6", preferIPv6String)
+                Config.replace("net_prefer_ipv6", preferIPv6String)
                 save = true
                 restart = true
             }