There's two ways bluetooth can be powered off/on for us: One way is to
go via airplane mode (which uses rfkill), and the second way is to tell
BlueZ to turn off the device. Now rfkill always has the final say on
whether bluetooth is off, BlueZ OTOH has the final say on whether
bluetooth is on.
This means when we want to know whether bluetooth is turned on, we only
have to ask BlueZ, so simply read this._client.default_adapter_powered
for that.
For turning bluetooth on or off we use rfkill, but when turning it on,
make sure it's turned on in Bluez, too.
FTR, this is exactly the same way the Bluetooth panel in Settings
handles this.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2188>
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