Add Data Saver to Firewall

[anubisproductions]

Adds Data Saver to Firewall.

Per-app mobile data measurement with per-network blocking - an app can be blocked on mobile data while left working normally on Wi-Fi. Blocking uses VpnService in package-list mode with no packet forwarding, so nothing is routed anywhere.

Against the criteria:

• FOSS / source available - GPL-3.0
• Keeps privacy - no ads, no analytics, no crash reporting, no accounts. The app ships without the INTERNET permission at all.
• No proprietary elements - no Google Play Services dependency, no proprietary SDKs.
• Stable and actively developed - released and maintained, currently version 0.8.
• Documentation - project site at https://anubisproductions.github.io/data-saver-usage-blocker/
• Free of charge - no purchases, no subscription, no locked features.

Not on F-Droid yet; the RFP is open at fdroid/rfp#4520, so the ent

[offa]

Thanks for your project. Since it is very new, let's give it some more time.

[anubisproductions]

That is fair, and thanks for saying so plainly rather than leaving it to rot.

Happy to close this myself if you would rather not carry an open PR for it - say the word, or just close it, and I will come back when the project has a track record worth pointing at rather than a promise. No need to reply if closing is easier.

[offa]

Feel free to leave this PR open :-)