Brightness: Migrate to deviceinfo instead of using libandroid-properties for ubuntu.widi.supported

Showing of the aethercast settings item (Brightness & Display > External display) is currently handled through value of ubuntu.widi.supported Android prop which is certainly unfriendly towards mainline devices running Linux as well and really feels like abusing the Android properties system as a form of configuration storage specifically for Ubuntu Touch applications which I personally don't like at all.

We'll also need to inform porters somehow that instead of setting ubuntu.widi.supported Android prop you now need to set some to-be-determined libdeviceinfo boolean; how should this be handled?

As discussed on 2022-08-16 development sync I'll leave the old ubuntu.widi.supported prop intact for now as well with a TODO to drop it with migration to e.g. 22.04 (I'll make a new issue about this afterwards). Also I'll comment on https://github.com/ubports/docs.ubports.com/issues/504 after the relevant MR is merged so we can keep track of the change and mention deprecation of ubuntu.widi.supported Android prop.