[java-logging] LoggingOptions.getDefaultInstance() always returns a new object

`LoggingOptions.getDefaultInstance()` always returns a new object:

```java
public static LoggingOptions getDefaultInstance() {
return newBuilder().build();
}
```

This is very misleading, because the method name suggests it will return the same instance.

Consequences are that `Logging` instance is not shared among handlers etc. if we use `LoggingOptions.getDefaultInstance()` to pass the options, without saving them to a variable.

In fact, our codebase had `LoggingOptions.getDefaultInstance().service` in a few places, and a new client was always created as a result.