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