VolumeControl: Early return in registerCallback

replaced:
```
if (service) {
  /*
  * long code
  */
}
```
by
```
if (!service) {
  return;
}
/*
* long code
*/
```

To win some indent spaces

Bug: 324492914
Test: m Bluetooth | Existing test
Flag: Exempt, mechanical no-op
Change-Id: Ia580c60c2e35b446ecdfb373afed10206e4376b7