Add Backup & Restore Metrics for Network Policies settings.

Bug: 379861078
Change-Id: If0e0f2d87d293830d23d3619d5a395225975b4d6
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.