Display eraser selection when using the hardware eraser

[olup]

Right now, the stylus' erase does work as expected, but has no visual feedback of the eraser's selection being made.
The quick implementation of the boox SDK (touchHelper) offers callbacks for the physical eraser, that are used in this codebase, but does not implement the drawing feedback for it (it seems - they expect a "live erasing" which is not what I'd like to achieve here)