Implement backup/restore for network policy

Modify network policy XML parser helper functions for backup/restore
scenarios, including secondary users

Test:
1. Block an app's (e.g. Chromium) wifi and mobile data through the firewall
2. Backup to internal storage on user 0
3. Create work profile
4. Copy .SeedVaultAndroid to the work profile (through its Files app)
5. Restore backup from work profile setup wizard
6. Verify the app's wifi and mobile data is still blocked

Issue: calyxos#2586
Bug: https://issuetracker.google.com/issues/207800947
Change-Id2: I44da196614a749c2c8b0113cade92215e88fed4a
Change-Id: I77158b344a47138a17fbce72d3ecb3be6927b870