bluetooth: report AG battery level

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.
2 jobs for native-backend-upower in 1 minute and 1 second (queued for 7 seconds)
latest
Status Name Job ID Coverage
  Container
passed build-container #14832513

00:00:11

 
  Build
failed build-meson #14832514

00:00:49

 
Name Stage Failure
failed
build-meson Build
[264/590] Generating symbol file 'src/25a6634@@pulsecommon-14.99@sha/libpulsecommon-14.99.so.symbols'.
[265/590] Compiling C object 'src/modules/9490802@@module-ladspa-sink@sha/module-ladspa-sink.c.o'.
[266/590] Compiling C object 'src/pulsecore/e9dd7ca@@protocol-native@sha/protocol-native.c.o'.
[267/590] Compiling C object 'src/modules/alsa/4616e5d@@alsa-util@sha/alsa-mixer.c.o'.
[268/590] Generating pulseaudio.desktop_src@daemon_merge with a custom command.
[269/590] Compiling C++ object 'src/modules/echo-cancel/5503603@@webrtc-util@sha/webrtc.cc.o'.
ninja: build stopped: subcommand failed.
Cleaning up file based variables
ERROR: Job failed: exit code 1