Add Backup & Restore Metrics for Network Policies settings.

This is a rollforward with fix of ag/31176152. The previous CL was causing crashes because I had forgotten to guard the new logic behind the flag. 

Bug: 379861078 
Change-Id: I37882ddef459e49087e6dd44c7fef8f91ef27238
Flag: com.android.server.backup.enable_metrics_settings_backup_agents
Tested: Tested manually, as there's no testing infra for these methods and I couldn't find a way to come up with one.