Fix NetworkMonitor won't do private dns resolution

NetworkMonitor cannot do the private dns resolution due to lack
"mNetworkCapabilities". ConnectivityService will call
NetworkStackService#notifyPrivateDnsChanged() before doing
network validation. After NetworkMonitor received
CMD_PRIVATE_DNS_SETTINGS_CHANGED, it will call
isValidationRequired to check if the validation is required.
But mNetworkCapabilities didn't be set, so the
mPrivateDnsProviderHostname will be set to empty and make
EvaluatingPrivateDnsState doesn't do private dns resolution.

Bug: 129375892
Test: 1. Build pass
      2. atest FrameworksNetTests
      3. atest NetworkStackTests

Change-Id: I637212fb4746bb2e9a2f4a4457fd36c4c2701149