Follow error prone recommendation

```
In ObexRejectServer.java
38: warning: [BadImport] Importing nested classes/static methods/static
fields with commonly-used names can make code harder to read, because it
may not be clear from the context exactly which type is being referred
to. Qualifying the name with that of the containing class can make the
code clearer. Here we recommend using qualified class: Handler
public class ObexRejectServer [...] implements Callback {
                                               ^
```

Bug: 236759221
Test: m RUN_ERROR_PRONE=true Bluetooth
Change-Id: I60c58853e0c0f2d30abdde4d4e4601477eb1dc6c