Add PrivateDnsCommon.h and PrivateDnsValidationObserver.h

1. Move some common enum to PrivateDnsCommon.h
2. Extract PrivateDnsConfiguration::Observer to
   PrivateDnsValidationObserver.h

Bug: 79727473
Test: cd packages/modules/DnsResolver && mm
Change-Id: I92cec1605c70250b3b79db5cb286d4e8933025ca
diff --git a/PrivateDnsConfiguration.cpp b/PrivateDnsConfiguration.cpp
index 472f1cd..cf8979d 100644
--- a/PrivateDnsConfiguration.cpp
+++ b/PrivateDnsConfiguration.cpp
@@ -330,7 +330,7 @@
     return false;
 }
 
-void PrivateDnsConfiguration::setObserver(Observer* observer) {
+void PrivateDnsConfiguration::setObserver(PrivateDnsValidationObserver* observer) {
     std::lock_guard guard(mPrivateDnsLock);
     mObserver = observer;
 }