There is only ever a single service to advertise via mDNS, so when a new
registration is requested, remove any existing ones. This should eliminate
these stacktraces:
java.lang.IllegalStateException: A service information can only be registered with a single instamce of JmDNS.
at javax.jmdns.impl.JmDNSImpl.registerService(JmDNSImpl.java:1005)
at org.fdroid.fdroid.localrepo.LocalRepoService$5.run(LocalRepoService.java:239)
at java.lang.Thread.run(Thread.java:856)