back button at main activity now behaves the same way as home button

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

@@ -371,6 +371,10 @@ class MainActivity : AppCompatActivity() {
         }
     }
 
+    override fun onBackPressed() {
+        moveTaskToBack(true)
+    }
+
     override fun onRequestPermissionsResult(requestCode: Int,
                                             permissions: Array<String>, grantResults: IntArray) {
         when (requestCode) {