refactor battery status to an optional characteristic

[ccutrer]

rather than have everyone implement an interface directly for every concrete accessory

also refactor in an AbstractSensor[Service] base class/interface so taht all the different sensors don't have to repeat their optional characteristics

[sobeos]

I would keep the architecture more open. Introducing a sensor interface would restrict the setup too much. I would introduce an interface for every characteristic, because there are numerous combinations. I understand the idea behind, but I fear it will not work for all accessories. Perhaps somebody has an idea to solve the problem. If not, I would copy the impl part for every class providing the optional characteristic. (most of the time not too many)