old dtmf watcher needs to be removed before new is added

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

@@ -1,5 +1,8 @@
 package com.tutpro.baresip
 
-class Call(val ua: String, val call: String, val peerURI: String, var status: String) {
+import android.text.TextWatcher
+
+class Call(val ua: String, val call: String, val peerURI: String, var status: String,
+           val dtmfWatcher: TextWatcher?) {
     var hold: Boolean = false
 }