[trel] updates for switching trel interface (#2579)

As it's possible to switch network interface for TREL in
openthread/openthread#10872, updated
trel_dnssd.cpp in 2 places:
1) Initialize() can be called multiple times now, so it should clear the
mTrelNetifIndex at the beginning to refresh the index according to the
given interface name.
2) HandleMdnsState() may be invoked when TrelDnssd is not initialized,
we should still update the mMdnsPublisherReady status, so it's ready to
publish a service when TrelDnssd becomes initialized.

src/trel_dnssd/trel_dnssd.cpp

@@ -91,6 +91,9 @@ TrelDnssd::TrelDnssd(Ncp::RcpHost &aHost, Mdns::Publisher &aPublisher)
 void TrelDnssd::Initialize(std::string aTrelNetif)
 {
     mTrelNetif = std::move(aTrelNetif);
+    // Reset mTrelNetifIndex to 0 so that when this function is called with a different aTrelNetif
+    // than the current mTrelNetif, CheckTrelNetifReady() will update mTrelNetifIndex accordingly.
+    mTrelNetifIndex = 0;
 
     if (IsInitialized())
     {
@@ -190,7 +193,6 @@ void TrelDnssd::UnregisterService(void)
 
 void TrelDnssd::HandleMdnsState(Mdns::Publisher::State aState)
 {
-    VerifyOrExit(IsInitialized());
     VerifyOrExit(aState == Mdns::Publisher::State::kReady);
 
     otbrLogDebug("mDNS Publisher is Ready");
@@ -202,6 +204,7 @@ void TrelDnssd::HandleMdnsState(Mdns::Publisher::State aState)
         mRegisterInfo.mInstanceName = "";
     }
 
+    VerifyOrExit(IsInitialized());
     OnBecomeReady();
 
 exit: