bluetooth: report AG battery level

🚨 PSA for distros 🚨

This MR is still in its early days, do not ship this to any branch of your distro! I also do not want any social media buzz or whatever. I put in a lot of work into this to get it in an upstreamable shape, so the least you can do as a distro is to be patient and give developers the credit they deserve ;) Toot is out to the general public: https://fosstodon.org/@dylanvanassche/108221554988020250 Distros & maintainers violating this disclaimer will be called out in public. You have been warned! 🔥

MR STATUS: reviewed, do not ship to any branch
I will update this description when the status changes regarding this.

Read more here: https://do-not-ship.it/

Report AG battery level to HF

oFono backend reports the AG's battery level to the connected headset, but this is not the case for the native backend. Add UPower backend to report the AG's battery when available.

This MR is initial work towards a better native bluetooth backend, I plan to add ModemManager as well for phone calls. It would be great to get feedback on my approach here and getting this merged before I work further on the ModemManager part as the approach would be largely the same, just different D-Bus interfaces and a lot more components than UPower.

I will continue testing this patch over the next few days and connect it to a car multimedia system to see how it behaves with such devices. Will report back here when done. Works fine on my car multimedia system :)

Contributes to #1217 (closed)

CC: @MarijnS95