Fix #36 - allow users to select 'none' for the preferred radio

If none is selected, we do not leave our service running.  And we do
not start it on boot.

app/src/main/java/com/geeksville/mesh/service/IRadioInterface.kt

@@ -4,4 +4,13 @@ import java.io.Closeable
 
 interface IRadioInterface : Closeable {
     fun handleSendToRadio(p: ByteArray)
+}
+
+class NopInterface : IRadioInterface {
+    override fun handleSendToRadio(p: ByteArray) {
+    }
+
+    override fun close() {
+    }
+
 }