Fix DiscoveryImplPlatform::Shutdown so restart works correctly. (#18542)

Until the changes in #18442 DiscoveryImplPlatform::Shutdown was a
no-op on most platforms, so the fact that you could not re-init after
that call did not matter, because nothing had actually been shut down.
Now that we are actually shutting down mResolverProxy, we need to make
sure a future call to Init() will correctly re-initialize things
instead of just bailing out.

src/lib/dnssd/Discovery_ImplPlatform.cpp

@@ -351,6 +351,7 @@ void DiscoveryImplPlatform::Shutdown()
  VerifyOrReturn(mDnssdInitialized);
  mResolverProxy.Shutdown();
  ChipDnssdShutdown();
+ mDnssdInitialized = false;
 }
 
 void DiscoveryImplPlatform::HandleDnssdInit(void * context, CHIP_ERROR initError)