Wouldn’t it make sense to allow hiding the top bar also if there is no top bar present and showing the menu button in the action bar like many apps do?
In addition, why not make the action bar hideable too? It might be shown by by some gesture like double finger tap.
That way you could use the full screen area for drawing.
Is there a way to implement support for stylus buttons? I have no Idea how that can be implemented in android applications.
I have got basuc android development skills and if you like, I will look into Implementing the proposed functions.